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Sustainable Water and Environmental Management in the California Bay-Delta (2012)

Chapter:4 Environmental Change: Challenges and Opportunities

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Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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4

Environmental Change:
Challenges and Opportunities

INTRODUCTION

Anthropogenic influences have rapidly and radically altered the bay-delta ecosystem over the past 150 years. Major changes such as land subsidence, climate change, habit alteration, water quality, population growth, water exports, invasion by nonnative species, and in-delta physical changes will continue to change the delta during the current century and beyond. Consequently, delta planning must envision a system that may be very different from what exists today, both physically and functionally. Rehabilitation planning in such a setting is extremely challenging as it is confounded by numerous uncertainties in the drivers of change. However, the projections of anticipated changes will allow many opportunities to tailor the restoration strategies to steer the future delta to a desirable state (Lund et al. 2010) and to include flexibility and wide tolerances in the design of water infrastructure and ecosystem rehabilitation. Some of the primary challenges include, but are not limited to, habitat loss, climate change including sea level rise, and levee stability. In this chapter, we discuss the details and the potential implications of these challenges and opportunities.

HABITAT LOSS

Habitat loss has been implicated as a major factor in species extinctions (e.g., NRC 1995, 1996, Seabloom et al. 2002). This relationship has been established over a very wide range of habitats and species, and there is no reason to conclude that it is any less important in the delta than elsewhere.

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

Indeed, the extent of changes in the delta (e.g., Lund et al. 2010; see discussion of changing delta environments below) compound the effects of the many dams on major delta tributaries that remove habitat for migratory species whose passage is blocked by the dams (e.g., NMFS 2009).

Habitat is the physical and biological setting in which organisms live and in which other components of the environment are encountered (Krebs 1985, NRC 1995). Thus, all aspects of the delta, past and present, serve as habitat and all the environmental changes described in Chapters 1 and 3 affect habitat and the species that depend on it. Many efforts have been made and are ongoing to measure and assess habitats in terms of their suitability for organisms (e.g., NRC 2008a). The habitats of the delta are diverse in character and include the water column; submerged substrates; adjacent intertidal, wetland, and upland areas; agricultural fields; levees; rivers and streams; the estuary; and so on. All of them have changed markedly in the past 150 years. Based on the complexity of delta habitats and the modifications to them, the interactions between stressors (for example, the interactions among temperature, salinity, and invasive cyanobacteria) must be considered.

In many cases, substantial knowledge exists around habitat needs for individual species. For example, much is known about what salmon need with respect to temperature, water flows and velocities, turbidity, water depths, substrate and gravel types, seasonality of many of the preceding factors, riparian vegetation, and especially access (e.g., see Williams 2006, McLain and Castillo 2010, NMFS 2009). For delta smelt, important habitat factors include open water, semienclosed bays, flow rates and volumes, temperature, turbidity, and salinity. The list of factors increases when habitat for their prey is also considered. Changes in pelagic fish habitat have been described (e.g., Nobriga et al. 2008). One key aspect for pelagic organisms is that, unlike species that require specific substrate conditions, high-quaity habitat (and, similarly, low-quality habitat) for these species shifts location with changes in water conditions, especially in tidal areas. Thus, management of the salinity gradient, for example, in the estuary has important implications for delta smelt and other pelagic species.

The delta ecosystem will never return to its predisturbance state. Changes in the template combined with changes in community composition provide a context for efforts to “restore” the delta. The changes in delta geometry in the past 150 years, in both vertical and horizontal planes, have resulted in a system dominated by subsided islands and deep, levee-bound channels. The continued loss of peat from the islands combined with rising sea level continues to lead the system away from its former topography and bathymetry (Mount and Twiss 2005). Recent studies (Brooks et al. 2012) point to subsidence of 3 to 20 mm per year associated with compaction of underlying Quaternary sediments. Brooks et al. conclude that “[b]y 2100,

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

all scenarios except the lowest rate [of sea-level rise] combined with the lowest reference frame bias project that at least ~38 percent and likely closer to ~97 percent of all levees” will subside by at least 0.5 m below their current elevations. In addition, the changes in water chemistry, nutrient concentrations, altered residence times, and their consequences challenge the re-creation of habitat. As an example, one of the challenges in rehabilitating the Everglades in Florida is that nonnative species, increased phosphorus loads, and changed hydrology mean that simply restoring water flow without other actions will not lead to a recovery of the former community structure and composition (e.g., NRC 2010).

Even if tidal water and dredged material were reintroduced to flooded islands to return them to an intertidal or shallow subtidal elevation, continued maintenance of such elevations in the face of sea level rise will be necessary to maintain native wetland plant communities within their hydro-logic tolerance limits and will require the accumulation of organic matter and sediment. Reed (2002) showed that even though delta wetland soils are frequently described as peats, the proportion of minerals in wetland soils even in the sediment-starved central delta was more than 75 percent on a dry-weight basis. Periodic inputs of sediments to the delta and redistribution of erodible material by tidal and flood flows were likely important in maintaining historic marsh elevations given underlying subsidence and sea level. However, Wright and Schoellhamer (2004) show that “the delivery of suspended sediment from the Sacramento River to San Francisco Bay has decreased by about one-half during the period 1957 to 2001.” They attribute this decline to many factors, “including the depletion of erodible sediment from factors that affect sediment load, including hydraulic mining in the late 1800s, trapping of sediment in reservoirs, riverbank protection, altered land-uses (such as agriculture, grazing, urbanization, and logging), and levees.”

Even if the historic mosaic of wetlands, mudflats, and shallow tidal channels could be re-created, changes in delta biological communities mean these habitats would likely be used by a different suite of species. Grimaldo et al. (2012) compared fishes caught in shallow subtidal areas in a remnant natural wetland with several areas returned to tidal action by inadvertent levee breaches. They conclude that physical habitat modifications and biological introductions have had irreversible effects on native fish assemblages and their habitats. Even in areas that had not undergone any physical modification to its historic marsh area, the subtidal mudflats surrounding the marsh were entirely colonized by invasive submerged aquatic vegetation (SAV) to the extent that it “choked out” any transitional open-water habitat between the shallow shoals and the marsh. The fish assemblage at the unaltered site in Grimaldo et al.’s study was dominated by introduced fishes, such as centrarchids, which are well adapted to SAV.

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

Recreating wetland-mudflat-channel configurations with land sculpturing may be possible, and reintroducing tidal flows to formerly isolated areas is a well-established restoration technique. However, a restored geosmorphic-hydrologic condition would not support the same assemblage of species in the same numbers as were present before the delta was altered, although it might be possible to approach previous community compositions in some places.

CLIMATE CHANGE AND THE DELTA ECOSYSTEM

Climate change is a challenge confronting the management and restoration of the Central Valley and bay-delta ecosystem. Future changes in the mean climate and its variability are expected to profoundly affect the physical and ecological structure of the ecosystem as well as the nature of water issues in California. The cascading effects of climate change begin with increasing temperature, which over the 50-year planning horizon of the delta is predicted to increase between 1ºC and 3ºC (Cayan et al. 2009). This equates to the mean annual air temperature in Sacramento increasing from the current 16ºC (~61ºF) to somewhere between 17ºC (~63ºF) and 19ºC (~66ºF). At first glance, this does not seem especially significant, since the average low temperature in Sacramento in December is 4ºC and the average high in July and August is 34ºC. However, accompanying a rising temperature, the pattern of precipitation and runoff is expected to change significantly and the sea level is projected to rise (USBR 2011). These factors will affect the bay-delta ecosystem, its tributary watersheds, and the water supply critical to both urban and agricultural users (Chung et al. 2009; USBR 2011).

Physical impacts of climate change in the bay-delta region have been well studied (e.g., Field et al. 1999, Cayan et al. 2008, Franco et al. 2008, CDWR, 2010, CAT 2010, USBR 2011). The work to date includes a systems approach for understanding the natural variability including the potential global teleconnections to the region’s climate (Redmond and Koch 1991, Greshunov et al. 2000), detection and attribution of historical changes in climate (Bonfils et al. 2008), quantification of potential changes in primary stressors of climate through analyses of the General Circulation Model (GCM) predictions (Cayan et al. 2009) and downscaling (Hidalgo et al. 2008. Maurer and Hidalgo 2008), impacts of projected sea level rise (Knowles 2009), and the sensitivity of the water resources system to climate change and sea level rise (USBR 2008, 2011). However, only a few projections have quantified the impacts of warming, consequent changes in hydrology, and the sea level rise on the ecology of the Central Valley–bay-delta region. Some initial work is under way to integrate links between climate, hydrology, and ecology in the bay-delta system and its watersheds

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

(CASCaDE 2010, Cloern et al. 2011), which should prove to be beneficial information for planners in the future.

In considering climate impacts on the ecosystem, the change and especially the variability in the seasonal patterns of precipitation, flows, and temperature are probably most important in disrupting the life history patterns of delta species. The delta is changing continuously and natural but extreme variations could pose significant threats to the sustainability of its desirable ecological functions.

A conceptual framework for addressing climate change effects in the bay-delta system includes the linkages between global drivers, both natural and anthropogenic, the regional and local stressors, and the corresponding effects. Warming due to anthropogenic greenhouse gases, as highlighted recently by the recent report of the Intergovernmental Panel on Climate Change (IPCC 2007), is the primary change in climate and the cause of sea level rise in the Central Valley. The other primary driver, natural variability, is manifested in multidecadal changes in precipitation and temperature patterns (Pagano and Garen 2005) and intradecadal variations associated with such phenomena as the El Niño/Southern Oscillation (ENSO) (Redmond and Koch 1991), the Pacific Decadal Oscillation (Francis and Hare 1994), and the North Pacific Oscillation (Pierce 2005). For example, Pagano and Garen (2005), who studied streamflows from 1901 to 2002 in California, showed that the period from 1980 to 2002 had the greatest variability and persistence in streamflows. This means that there were periods of wet years along with multiyear extreme droughts. El Niño winters result in wetter winters, particularly in South California, but have had a lesser impact on northern regions of the state (Redmond and Koch 1991, Cayan et al. 2009). Ocean-atmospheric patterns will also elevate the sea levels along the west coast during the El Niño years (Cayan et al. 2008).

In the ensuing sections, we begin with a review of the magnitude of climate change and sea level rise and large-scale hydrologic effects of climate change, scale down to how changes may disrupt the life cycles of listed delta species, assess how these effects might impact restoration planning efforts, and finally provide suggestions for dealing with climate change.

Estimates of Climate Change

Temperature and Precipitation

Results of climate modeling are not necessarily accurate predictions of the magnitude of warming. However, model projections consistently show that the gradual warming in California during the earlier part of the 21st century is very similar for various emission scenarios, but they may differ in the later decades. Projection estimates vary but the midcentury warm-

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

ing is in the range of 1ºC to 3ºC, which will increase to 2ºC to 6ºC by the end of the 21st century (Cayan et al. 2009). Climate models also predict substantial variability in warming across the Central Valley (USBR 2011). This asymmetry in temporal (both seasonal and decadal-scale) and spatial warming will substantially affect precipitation patterns (snow versus rain), snowpack, and the snowmelt in the tributary watersheds of the bay delta. Compared to the historical period, spring temperatures are projected to be warmer, particularly during the second half of the century, and reduce April 1 snowpack, a key indicator of water supply for the following summer and fall. The duration of extreme warm temperatures grows from 2 months (July-August) to 4 months (June to September) (Climate Action Team Report 2010). Heat waves are also projected to increase in frequency and magnitude.

Projections indicate that precipitation may decline in some regions of the Central Valley, particularly during the mid- to late 21st century (Cayan 2009, USBR 2011). They also show that precipitation may increase slightly until the middle of the century, which may be followed by a decline during the later part of the century. Although precipitation predictions are highly uncertain (Chung et al. 2009), projections of increases in temperature, predicted by all models, are more certain. The effect on snowpack and snowmelt of these projected temperature increases would be a significant change in the timing and magnitude of flows in the tributary rivers of the bay-delta system (USBR 2011).

Sea Level Rise

Sea level rise driven by global-scale climate change will affect, perhaps irreversibly, the bay-delta hydrodynamics, levee stability, and salinity conditions (Mount 2007, Lund et al. 2010). Higher ocean levels, particularly in the presence of tides, and storms, which may be exacerbated by ENSO conditions, will increase water depths and push salty water further inland, affecting vertical mixing. The exact effect of sea level rise depends on its magnitude. The historical rate of sea level rise at the Golden Gate is estimated to be about 2 mm/yr (equivalent to about 0.2 m over the 20th century).

During the 20th century, the global mean sea level rise has been estimated to be about 1.7 mm/yr (Church and White 2011). IPCC (2007) projected the sea level rise by 2100 to be in the range of 0.18 to 0.59 m but it did not include possible rapid changes in ice sheet dynamics. The current research suggests that, during the 21st century and beyond, sea level rise may accelerate, but the estimates of the rate of acceleration vary as indicated by the wide range of sea level rise suggested for 2100 in the literature. The uncertainties in projections have been attributed to the difficulties in

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

projecting the melt rate of land-based ice, particularly in Greenland and Antarctica. Temperature-based projections (Rahmstorf 2007) suggest that the global mean sea level rise may be as much as 1.4 m or more (Pfeffer et al. 2008, Vermeer and Rahmstorf 2009). Clearly the magnitude of the future global sea level rise is uncertain but the range 0.18-1.4 m or the sea level rise that has been suggested by USACE (2011) should be useful for scenario planning in restoration efforts (e.g., Heberger et al. 2009, 2011).

Effects of Climate Change on Delta Hydrology

Climate change could have a variety of impacts on both natural and human systems in the bay-delta region. In terms of hydrologic changes, one of the key outcomes of warming will be to alter the temporal patterns of precipitation and tributary runoff. Under warmer conditions, precipitation during the winter will occur more as rain instead of snow and, as a consequence, the April 1 snowpack will decline (Mote et al. 2005, Knowles et al. 2006, Chung et al. 2009, USBR 2011), which will reduce the summer low flows (Maurer 2007). The modeling results indicate that the runoff resulting from increased rain during the winter months of December through March will increase during the 21st century (USBR 2011). However, the snowmelt runoff from tributaries during the April-July period will decrease with larger magnitudes expected during the later part of the 21st century. Such significant changes in the magnitude and timing of runoff into major reservoirs in the Central Valley could have important impacts in terms of reduced storage opportunities, less year-to-year carryover storage, and less water for cold-water releases during the hot summer months (USBR 2011).

Unless changes to the operational rules are made, the increased runoff into major reservoirs in the tributary watersheds during winter months may have to be released earlier for flood protection. This would in turn reduce the amount of storage available to meet the demands during the following summer and fall. The recent records already show changes in timing of flows from the headwaters of the Sierra Nevada region (Dettinger et al. 2004, Knowles and Cayan 2004, Stewart et al. 2004, Vicuna and Dracup 2007, Kapnick and Hall 2009). With high confidence, it can be concluded that the future temperature increases will continue to cause changes in streamflow timing and such projected changes will exceed those from natural variability (Knowles and Cayan 2002, Maurer et al. 2007). For example, Chung et al. (2009) have shown that in case of a 4ºC warming scenario, the average day by which Lake Oroville receives half its annual inflow shifts from mid-March to mid-February (about 36 days) and that the annual runoff fraction during the snowmelt period of April through July will decrease from about 35 percent to about 15 percent.

Warming has the potential to increase evaporative losses from both

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

soils and water bodies and as a consequence increase water demands of both agriculture and landscape irrigation. Increased CO2 will have complex interactions among processes affecting evapotranspiration from plants. Baldocchi and Wong (2006) have suggested that warming effects on agriculture may include the lengthening of the growing and transpiration seasons of the crops and a reduction of winter cold affecting fruit species. Groves et al. (2008) determined that climate change could increase the outdoor water demand by up to 10 percent by 2040 and decrease local water supply by up to 40 percent. With a decrease in spring and summer runoff, the difference between supply and demand will grow at a faster pace. Climate change will require a change in future operation and planning of water resources systems and the current regulatory policies (Willis et al. 2011).

In a widely quoted paper, Milly et al. (2008) claimed that the traditional “stationarity” assumption used in planning of water resources projects was no longer viable or prudent. The changes in hydrology described above would pose significant challenges for the management of the water resources systems such as the Central Valley Project (CVP) and the State Water Project (SWP). Willis et al. (2011) suggested that the “static” rules curves that exist today may perform poorly under the climate change scenario and that more flexible dynamic operating rules may be needed in the future (see Trimble et al. [2005] for an example of such rules). The U.S. Bureau of Reclamation in its 2008 Biological Assessment analyzed the sensitivity of future state and federal projects in the bay-delta region to potential climate change and associated sea level rise (USBR 2008), finding that CVP/SWP deliveries and carryover storages were sensitive to precipitation changes and sea level rise would lead to great salinity intrusion into the delta. Increased air temperature would reduce the cold-water storage of the reservoirs and increase temperature regimes of the major tributaries of the delta, which in turn would affect the survival of both delta smelt and salmon. The study also indicated that the negative flows in the Old and Middle rivers will increase under climate change scenarios, primarily during the winter, exacerbating fish entrainment at the CVP/SWP pumps. However, the study also found that uncertainty in precipitation projections makes it difficult to assess the level of impacts, as a potential increase in precipitation may offset the warming impacts.

The Department of Water Resources conducted a separate modeling study to investigate the effects of climate change on both the federal and state water projects (Chung et al. 2009). The results (Table 4-1) suggest that the SWP/CVP water supply reliability would be affected significantly under the projected climate change scenarios. Reduction in delta exports to the Central Valley was predicted to be in the range of 7 to 21 percent and the water supply deficit in the south, resulting from such conditions, would likely be met by increased groundwater mining, exacerbating the current

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

TABLE 4-1 Summary of Water Resources Impacts Considering 12 Future Climate Scenarios

  Midcentury: Some Uncertainty End of Century: More Uncertainty
  Lower to Higher GHG Emissions Lower to Higher GHG Emissions
Delta Exports – 7 to –10% –21 to –25%
Reservoir Carryover Storage –15 to –19% –33 to –38%
Sacramento Valley Groundwater +5 to +9% +13 to +17%
Pumping
CVP Generation –4 to –11% –12 to –13%
CVP Use –9 to –14% –24 to –28%
SWP Generation –5 to –12% –15 to –16%
SWP Use –5 to –10% –16%
X2 Delta Salinity Standard Expected to be met Expected to be met
System Vulnerability to Interruptiona 1 in 6 to 8 years 1 in 3 to 4 years
Additional Water Needed to Meet Regulations and Maintain Operationsb 750 to 575 TAF/yr 850 to 750 TAF/yr

NOTE: CVP, Central Valley Project; GHG, greenhouse gas; SWP, State Water Project; TAF, thousand acre-feet.

a The SWP-CVP system is considered vulnerable to operational interruption during a year if the water level in one or more of the major supply reservoirs (Shasta, Oroville, Folsom, and Trinity) is too low to release water from the reservoir. For current conditions, the SWP-CVP system is not considered vulnerable to operational interruption.

b Additional water is needed only in years when reservoir levels fall below the reservoir outlets.
SOURCE: Chung et al. (2009).

problem of declining groundwater levels in the Central Valley (Famiglietti et al. 2011). Reservoir carryover storage, the quantity of water available on September 1 for improving water-supply reliability during the ensuing year, is expected to decline by 15 to 38 percent depending on the climate change scenario. Significantly, the study indicated that in some years the water levels in reservoirs may fall below the lowest release outlets leading to operational interruptions, which may occur as frequently as once every 3 years (Table 4-1). In spite of the water shortages, the CVP/SWP system was expected to meet the delta salinity standard related to the position of X2 (“delta salinity standard”). Other modeling suggests that there is considerable physical and economic flexibility in the system, although at some cost (Tanaka et al. 2006, Harou et al. 2010, Buck et al. 2011). This flexibility likely will be needed to adapt to future conditions.

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

Effects of Sea Level Rise

Ecosystems physically connected to the ocean, such as the California bay-delta system, will have compounding effects of climate change due to accompanying sea level rise on both global and regional scales. Increases in ocean levels at the mouth of the San Francisco Bay will have significant impact on the upstream regions of the bay as well as the delta. A larger concern is the changes in the sea level extremes, which are exacerbated not only by the mean sea level, but also by astronomical tides, winter storms, and the presence of large-scale ocean phenomena such as El Niño. Predictions of the changes related to additional factors are uncertain but it is likely that today’s extremes experienced by the bay-delta system will become more frequent.

As discussed in the next section, the projected changes in both the average and extreme sea levels in the interior of the delta may significantly affect the structural integrity of levees protecting delta islands. In view of the changes in the tidal fluctuations, particularly during storms, the frequency of levee failures and the flooding of delta islands are likely to increase. Historical efforts to control floods do not appear to have reduced the levee failure frequency (Florsheim and Dettinger 2007). The frequency of levee failure is likely to increase in the future with potential increases of flood flows from the upstream reservoirs as a result of timing change in runoff and increased water levels in the delta conveyance canals due to sea level rise. The dual effect of sea level rise and the increased flood flows will be largest when the astronomical and weather factors (e.g., high tides and sea level increases due to storms and teleconnections such as El Niño) and the peak discharges from the upstream coincide to create a rare combination of factors affecting the water levels in the bay and delta. Levee failures will flood delta islands, either permanently changing the geomorphology and the habitats of the delta system or requiring massive investment to reestablish the status quo. It has been suggested that restructuring of bay-delta habitats as a result of levee failure could increase habitat diversity, expand flood-plain area, and increase extent of open-water habitats. Such changes could improve conditions for some desirable delta fish species (Moyle et al. 2010).

Another effect of sea level rise will be increased saltwater intrusion into freshwater parts of the delta system. When saltwater intrusion occurs in the interior parts of the delta, quality of water that is exported will degrade significantly and aquatic habitats will shift or may be eliminated entirely. Frequent interruptions of water supply to the south via the export pumps will clearly pose problems for providing adequate water supply for farmers and the urban users in Southern California (Medellin-Azuara et al. 2008, Chen et al. 2010). The ultimate result will be for the users south of the delta to depend on more and more groundwater supplies in the regions

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

to the south, which have already been mined through excessive pumping (Famiglietti et al. 2011). Permanent changes to the salinity levels in delta channels will also degrade the quality of water that is used for agriculture and other uses within the delta islands.

Climate Change Effects on Water Temperature

The water temperature in the delta and upper San Francisco Bay varies considerably through the year with a range of 7ºC to 30ºC (see Figure 4-1). While temperatures primarily vary seasonally, as seen in Figure 4-2B below, temperatures on any given day can be several degrees warmer or colder than the seasonal average.

At any point in the system this temperature reflects the combined effects of solar insolation, surface heat exchanges, river flow, and dispersion, as well as the temperatures in the rivers upstream and ocean downstream (Monismith et al. 2009). To examine the potential effects of climate change on delta temperatures, Wagner et al. (2011) created a statistical model based on fitting 10 years of data using an autoregressive model for daily water temperature as a function of air temperature and solar insolation. On the basis of this model, Wagner et al. argue that the effects of flow are generally small and are confined to shorter time scales, and so could be ne-

image

FIGURE 4-1 Suisun Bay delta water temperature for the period 2000-2006.
SOURCE: Data from California Data Exchange Center.

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

glected, at least when considering climate effects. In particular, the residuals of their model are weakly correlated (San Joaquin River) or uncorrelated (Sacramento River) with flow. This is plausible given that travel distances between sources of cold water (i.e., reservoirs) and the delta may be sufficiently great for river temperatures to approach the equilibrium temperature, that is, the temperature at which the net heat flux is zero (Mohseni et al. 1999). Additionally a statistical regression model, developed under the CASCaDE (2010) project, demonstrated some promise for predicting water temperatures in the delta.

Climate Effects on Species

To understand the effects of climate change on the delta fish species, we need to consider the effect of climate change on the intensity and duration of summer heat, the frequency of floods and multiyear droughts, and the level of snow pack. In short, while climate change is typically described in terms of trends in mean weather patterns, what matters to fish are the frequency and intensity of extreme events that can disrupt their life history strategy and ultimately their survival. Here we illustrate how climate events will challenge fish.

Salmon

Salmon and steelhead are poikilothermic (cold-blooded) animals that thrive over a wide range of temperatures. However, at the upper limit of the range (~22ºC), their respiration increases, growth declines rapidly (Figure 4-2A), and they become susceptible to infection: all factors that increase mortality. Central Valley salmon experience nearly 100 percent mortality when temperatures exceed 23ºC (Baker et al. 1995). In the Central Valley, summer water temperature regularly exceeds salmons’ threshold (Figure 4-2B); consequently, heat-avoiding strategies have been selected for. Fall/ late-fall runs of Chinook salmon have the most straightforward strategy, which is simply to avoid the Central Valley in the summer. The adults enter the valley in the autumn and move quickly into tributaries to lay their eggs. The juveniles hatch in the winter and spring and leave the Central Valley before the summer. More complex strategies involve either eggs or adults being in cool-water refuges in the summer. For the winter-run Chinook salmon, the adults enter the valley in the winter and move to the upper Sacramento where they spawn in the summer, in fractured basalt habitats fed by cool-water springs. The juveniles emerge from the gravel in the fall and migrate downstream in the winter. For spring-run Chinook salmon, the adults enter the Central Valley in the spring and migrate to high-elevation tributaries where they “oversummer” in cooler waters. When the temperature drops in the autumn, they move into spawning habitats in the streams.

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
×

image

FIGURE 4-2 (A) effect of temperature on growth of Chinook raised at various temperatures for 28 days. From Williams (2006) redrawn from data in Brett et al. (1982). (B) Daily minimum, maximum, and average water temperature in the northern delta (Sacramento River at Freeport, near Sacramento). In panels A and B the red lines demark the temperature (~22o C) above which salmon growth declines and the blue lines depict the lethal temperature threshold (24o C) for salmon. SOURCE: Williams (2006).

The juveniles emerge in the winter and migrate downstream before the onset of summer. In essence, each run exploits a spatiotemporal window of opportunity within the Central Valley. The strategies allow what are essentially cold-water species to occupy warm-water habitats. However, these habitats are the southern boundary of salmon. Windows of opportunity for avoiding lethal temperatures do not exist south of the Central Valley. Furthermore, human development has significantly reduced the limited windows of opportunity that did historically exist here. Dams block access of spring Chinook to the most high-elevation habitats, as well as access of winter Chinook to cool groundwater habitats and access of fall Chinook to tributaries (Lindley et al. 2006). How climate change will affect these windows is highly relevant in the rehabilitation of Central Valley salmon and steelhead. The effects of climate change on phenotypic plasticity and evolution, and their implications for population persistence (survival), are discussed by Reed et al. (2010).

To explore how the windows will change, consider a scenario in which Central Valley streams warm by 1ºC, which is a reasonable midcentury estimate (Wagner et al. 2011). The average daily temperature would become what is now the maximum daily temperature line in Figure 4-2B. This temperature increase would increase mortality for outmigrating spring-run salmon smolts. Supporting evidence includes (1) observations of increased mortality in salmon at high temperatures (Williams 2006), (2) observations (1976-1981) of a strong negative correlation between June smolt survival

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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and average June water temperature at Sacramento (Kjelson et al. 1982), and (3) a statistical study revealing the importance of temperature on smolt survival in the delta (Newman and Rice 1997).

Not only are summer temperatures expected to become more lethal to fish by midcentury, the number of months with high temperatures is expected to double or more (Wagner et al. 2011). Expanding the duration of high temperature would narrow the window in which fall Chinook runs could occupy the Central Valley, which would disrupt both their growth pattern and migration timing. At the very least, the population would undergo a period of rapid selection under a new temperature pattern (Crozier et al. 2008). Hotter summers would increase the temperature of high-elevation streams and reduce, or intermittently remove, the cool-water habitat spring Chinook seek in the summer, a situation already observed in the Central Valley (Williams 2006). Winter Chinook runs would also be affected. Using the statistical life-cycle OBAN1 model calibrated with recruitment and environmental data between 1967 and 2008, Lessard et al. (2010) found that egg-rearing temperature of winter Chinook above Red Bluff Diversion Dam was a major determinant of year class strength. Furthermore, climate models predict increased variability in climate extremes, so infrequent but intense summer heat waves could have even greater effects on sensitive life history strategies.

Climate change models also predict increased duration and intensity of droughts in the western United States (CCSP 2008). Here the potential effects on salmon would be uniformly negative. Summer flows in high elevations would be reduced, affecting adult spring Chinook; summer groundwater flows would be reduced, affecting winter Chinook eggs; and autumn and spring flows in tributaries would be reduced, affecting growth opportunity and migration timing of fall/late-fall Chinook. Droughts extending over multiple years are of particular concern because several brood years would be affected, thus reducing the natural resiliency salmon obtain by intermixing fish from different brood years on the spawning grounds. Another aspect of the predictions of climate change models is more-intense precipitation events and floods (Min et al. 2011), which can scour the gravels, killing fish eggs while they incubate.

Finally, climate change can affect salmon indirectly through its effect on coastal winds, which drive coastal upwelling that fertilizes the food web on which salmon depend when they enter the ocean (Lindley et al. 2009). Furthermore, because large-scale climate patterns affect both the freshwater and ocean habitats of salmon (Lawson et al. 2004), extreme stream temperatures and reduced coastal winds could act together to amplify the impacts of climate change.

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1 Oncorhynchus Bayesian ANalysis

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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Thus, even though summer temperatures in most of the Central Valley streams are lethal to salmon, the fish exploit windows of opportunity for when to enter the Central Valley and where to spawn, so that they and their offspring avoid the high temperature. However, over the 21st century, if predictions of warmer and longer summers and shifts in precipitation and coastal winds come true, the windows of opportunity for many runs will narrow and some will eventually close. Furthermore, the process is not expected to be gradual. The frequency and intensity of daily and seasonal weather extremes will exceed the historical levels in which the salmon evolved their current life history strategies. The consequences are potentially great because even now Central Valley salmon live at the threshold of their temperature range. For the most part, studies of the impact of climate change have been cast in the context of mean trends, not in terms of changes in extremes. For example, a conceptual model was developed to explore possible evolutionary responses of Pacific Northwest salmon life history and tolerance to heat with changing environmental conditions (Crozier et al. 2008). Such evolutionary-scale focuses are highly relevant to the long-term effects of climate change on fish, but the more immediate issue, especially for the Central Valley, which is the southern end of the salmonid range, is the impact of extreme events such as heat waves and multiyear droughts. Indeed, demonstrable effects of climate events may already have occurred as evidenced by the 60 percent summer mortality of spring Chinook in Deer Creek in 2002 (Williams 2006) and the failure of the 2004 and 2005 fall Chinook classes because of the collapse of the Gulf of Farallones food web (Lindley et al. 2009).

Delta Smelt

The effects of temperature may be more critical for delta smelt than for other fish in the basin. In his assessment of the state of the delta smelt population, Bennett (2005) notes that few delta smelt were caught in any of the various surveys when the water temperature exceeded 20ºC. Moreover, lab studies cited by Bennett find that spawning was confined to temperatures between 15ºC and 17ºC, whereas an optimal range for spawning determined from field observations of larval delta smelt distributions appears to be 15ºC to 20ºC. Additionally, Bennett (2005) found that a significant correlation exists between delta smelt abundance and the length of time that the water temperature in the delta was between 15ºC and 20ºC. Finally, Swanson et al. (2000) found that temperatures over 25ºC are lethal for delta smelt. Importantly, using downscaled predictions of atmospheric temperature from several GCMs, Wagner et al. (2011) projected that the delta can be expected to warm by several degrees over the next century. As a result, this will shift the window in time when temperatures are suitable for delta smelt spawning 2 weeks earlier in the year and will mean that large

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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portions of the delta will be lethal for delta smelt for a significant portion of the year (10 to 60 days; see their Figure 12).

Other Species

Besides salmon and smelt, it is likely that temperature will affect several other organisms, including the listed green sturgeon. However, to the best of our knowledge, there are no studies of temperature effects on other organisms, e.g., benthic infauna like the invasive clam Corbicula fluminae, or the various zooplankton species resident in the system. Increases in water temperature might produce more subtle food-web changes. For example, since growth rates of cyanobacteria like Microcystis aeruginosa increase substantially with temperature, a warmer delta might be more prone to Microcystis blooms that could reduce production of phytoplankton that are more easily grazed and made use of by larval fish or by zooplankton that make up the prey of juvenile and adult fish (Lehman et al. 2005). Evidence for temperature related shifts in phytoplankton community structure in the delta is given by Lehman and Smith (1991) and by Lehman et al. (2008).

Managing Temperature

From the perspective of water resources management, it does not appear that the increasing delta water temperatures can be efficiently mitigated by project and reservoir operations. In principle, delta water temperatures can be affected by river flow rate and reservoir release temperature, since flow determines the time required for water to travel between reservoir outlets and the delta and hence the time over which radiative heating and heat exchanges with the atmosphere can raise the water temperature to the equilibrium temperature2 (Monismith et al. 2009). Thus, the farther the reservoir is from the delta, the less effect a given flow rate has on the temperature since the longer it takes for water to travel to the delta, the more likely it will be close to the equilibrium temperature (Deas and Lowney 2000). The farther the reservoir is from the delta, the more flow it takes for a given release temperature to produce a desired temperature at the delta.

Looking at data relating water temperature at Vernalis for a critical window (April 15 to May 13) of salmon smolt outmigration in the San Joaquin River, Cain et al. (2003) suggested that a flow of 5,000 cubic feet per second (cfs) in the main stem of the San Joaquin was needed to provide water temperatures suitable for salmon migration, although this correlation may mask the effects of other variables like air temperature. In contrast, the statistical model of Wagner et al. (2011), suggests little influence of flows.

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2 The equilibrium temperature is the temperature at which the net heat flux into the water is zero. It is generally above the atmospheric temperature.

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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Most reservoirs, particularly the largest ones, Shasta and Oroville, are too far upstream or have insufficient water in the cold water to affect delta temperatures significantly.

Thus, while further work to understand the linkage of flow, and hence water operations, and delta water temperatures (e.g., along the lines described by Deas et al. [1997]), may refine the picture presented above, the committee concludes that it is unlikely that reservoir releases can be effectively used to control delta water temperatures.

Integrating the Analyses

The preceding discussion suggests that many variables and factors need to be considered in projecting the effects of climate change on the Central Valley system. In such a situation, an integrated analysis using a series of linked models would be required to understand the cascading effects and the feedbacks on the large water resources system, including the delta. Since comprehensive biological models are not available for analyzing how climate change and sea level rise may affect species in the greater ecosystem, many attempts have been made to project indicators of hydrologic and ecological changes that may result from a range of climate change scenarios. Cloern et al. (2011) present such an analysis using nine indicators of changing climate, hydrology, and habitat quality where projections were made using a series of linked models for simulating meteorology, hydrology, sea level rise, estuarine salinity, sediment transport, and water temperature. This type of analysis, where alternative scenarios are used to link climate change to hydrologic and then biological processes, is extremely useful for understanding the range of changes that may be expected and planning future strategies for dealing with climate change. An example would be a life history model for San Joaquin salmon abundance based on flow and water temperature; the hydrologic parameters would be driven by climate change projections. Depiction of changes in the form of decadal trends, as shown in Figure 4-3, is useful for bracketing the future changes that may be expected in key indicators important for the development of response strategies. The range of climate change impacts discussed in the preceding sections, primarily in the form of increased air and water temperatures, less precipitation, higher sea levels, reduced runoff and late spring snow pack, and increased salinity, is similar to that estimated by Cloern et al. (2011) and shown in Figure 4-3. The committee concludes that the type of analysis conducted by Cloern et al. (2011) is extremely important to understand interacting effects and encourages the agencies to continue to improve their approach by adding other models such as those designed to predict species response.

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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FIGURE 4-3 Projected 2010-2099 changes in selected environmental indicators expressed as median trend per decade for two climate scenarios (red and blue). Statistically significant (p < 0.05) trends are indicated with solid circles and the horizontal lines show the 95 percent confidence limits for the trend estimates. SOURCE: Reproduced from Cloern et al. (2011).

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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Dealing with Climate Change

Because extreme events, whether they are from floods, droughts, or heat waves, will have large effects on fish and other delta populations, the adequacy of restoration actions and population models need to be considered in the context of increasing frequency and intensity of events. For example, if the frequency of extended hot dry summers increases, the frequency of year class failures would increase and the probability of extirpation of several salmon runs would increase. However, projecting the impacts of a changing frequency of extreme events is difficult. Current life-cycle models (e.g., Holmes 2001, Hinrichsen 2002, 2009, Lessard et al. 2010) assume that the pattern of demographic variability in the population is stable into the future. With climate change this assumption is violated, as described by Thompson et al. (2011). Furthermore, models are sensitive to choices of parameters characterizing future trends and so those not validated with data should be used with caution (Hulme 2005). In spite of these limitations, models linking climate variability and fish ecology are essentially the only way to project future impacts of climate change on fish (Jackson et al. 2009). However, those models need to be tested by careful monitoring; some effects of climate change on fish can be tested experimentally.

Information on climate science shapes public opinion regarding climate change, and the studies have much to contribute to the adaptive management of the Central Valley. While individuals typically form opinions either by learning from experience or from descriptions, experiential learning is the most compelling. However, when climate change is gradual, it has not been very noticeable to the public (Weber 2010). However, extreme events and resulting fishery closures are directly experienced by the public and noticed, although the public will not necessarily notice a connection between extreme events and long-term change. Yet if the climate predictions are correct, frequent extreme events will increase the need for Central Valley water resources by both the ecosystem and the public. In this case, managers may be asked to consider hard choices that are more in the context of triage than rehabilitation (e.g., CASCaDE 2010, Hanak et al. 2011, SPUR 2011). While such a scenario may not come to fruition, the committee encourages continued critical and comprehensive studies of the full range of future possibilities and how to adapt to climate change. Indeed, the committee recommends this kind of approach to delta issues in general.

In the future, effects of climate change will increase the need for Central Valley water resources by both the ecosystem and the public and induce even more competition among them. In developing alternative scenarios for the implementation of water management measures, it will be necessary to consider a larger variability in water supply and potential impacts on the ecosystem.

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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Incorporating climate change requires adoption of a nonstationary view; in other words, it reuires recognition that environmental correlates of climate continually change (Milly et al. 2008). Public investments and habitat management or water conveyance facilities should be evaluated and ranked for their adaptability to anticipated changes. In view of many uncertainties in the future extent of climate change, integrated analysis should be followed up with adaptation strategies using scenario planning and risk management strategies (e.g., Linkov et al. 2006). An approach that does not consider alternative futures may fail to achieve the anticipated benefits leading to the further degradation of the bay-delta ecosystem. Sustainability planning efforts will be successful only if they address the above challenges and the associated uncertainties. In the light of potential increases in water shortages and the competition for water, the committee judges that there are many opportunities and basic strategies for ensuring the long-term sustainability of the bay-delta ecosystem. Key considerations are discussed below.

Demand Management

Targeted reduction of demands through water conservation, and changes to the system of water rights and marketing, and alternative water supply (e.g., reuse), improvement of water use and conveyance efficiencies should be considered as integral components of future plans.

Increased Storage

Restoration of variability in flows that has been lost due to water management, hydrologic changes due to climate change, and the increased demands may require flexible operating strategies and increased water storage. In particular, anticipated changes in timing and magnitude of inflows may require additional storage in the system in order to meet the deficits in water supply, restore cold-water pools, and carry over storage in the system. Furthermore, additional flows may be required to mitigate impacts of saltwater intrusion and upstream migration of the delta X2 salinity standard during droughts. Groundwater storage with artificial recharge, particularly in the Central Valley south of the delta, should be considered, along with opportunities to increase reservoir storage in the system or to change the operating rules for existing systems. The expansion of storage should not come at the expense of negative environmental impacts, and comprehensive planning investigations will be needed to explore this option (Medellin-Azuara et al. 2008, Tanaka et al. 2008, Harou et al. 2010). Another example, which is nonstructural, is the reestablishment of Environmental Water Accounts, a measure that had been used in the past.

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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Operational Flexibility

Water exports through the current CVP and SWP systems during dry periods through delta channels have shown to be harmful for delta smelt and other resident fish species. Conveyance of water through the delta is likely to experience additional constraints due to climate change. Flexibility in operations achieved through the establishment of multiple conveyance routes and operation of the water storage with foresight (e.g., based on climate outlooks) should help reduce the harmful effects of constraints and competition among urban and agricultural users and the ecosystem. However, a strong regulatory framework will be needed to ensure that the increased flexibility is not used to favor one user type over the other.

Establishment of Environmental Flows

Climate change will increase the competition for water among the users. Maintaining the flows necessary to sustain the protected species in the delta likely will require establishment of minimum flows but, more important, will require consideration of the timing, frequency, duration, and magnitude of flows and the rates at which those flow parameters change. Establishment of such flows will require a careful analysis of environmental water needs, water availability during droughts, development of water shortage policies, and the implementation of specified conveyance priorities.

The committee has identified a variety of tools for predicting the effects of climate change on the key variables that will affect the bay delta. Since climate change is expected to alter these variables in new ways, extrapolations of historic data (stationarity) are not a sufficient basis for future public policy. A new combination of predictive models and data that defines actual changes as they occur is needed to assess risk and make investments. State and federal agencies have done much to translate climate change models into predictive regional effects in California and in the bay-delta area. But risk analysis is needed to provide a justification or rationale for public investment. Small investments such as research and data gathering, and some forms of demand management, should not require a high level of confidence that a particular situation will occur. However, if a proposed policy or action is very costly, more confidence that it will actually achieve its purposes will be needed.

This committee’s assignment has been focused on the bay-delta environment and water quantity and quality issues. These issues are but part of a larger picture of public investment in anticipation of the continued generation of environmental predictions. The 0.2-m rise of San Francisco Bay during the 20th century did not require large additional flood protection works. However, if the 21st century should see a significant increase in this trend, major investments will be needed in the bay delta to mitigate the broader impacts of climate change. In principle, this will be true with

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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regard to water management practices in the bay delta and throughout the state. A significant effort is needed to develop new tools to assess risk, and to provide the public justification necessary to support the major public and private investments that will be needed.

LEVEES

The hydrology of the delta is profoundly influenced by its 1700 km of levees (CDWR 1995), with its ecology and services described as “levee-dependent” (Lund et al. 2010). Indeed, the levees play a critical role throughout California in reducing flood risk, supporting agricultural production, and providing reliable water supply.

However, despite their importance, the levees of the delta are broadly vulnerable to failure associated with seismicity, flooding, subsidence, seepage, and sea level rise (CDWR 2009). Levee failures have occurred regularly throughout the Sacramento–San Joaquin system since the first breach in 1852, with breaks occurring during 25 percent of years (Florsheim and Dettinger 2007). However, it has been the more recent failures that have acutely revealed the vulnerability of levees in the delta and directed attention toward the likelihood and consequences of their breaching. Beginning with the 1986 flooding in the Central Valley, followed by major levee failure in the 1993 Missouri River floods (Tobin 1995), through levee failures at Mildred Island in 1983, Liberty Island in 1998, and Lower Jones Tract levee in 2004, and with catastrophic breaches in New Orleans during Hurricane Katrina in 2005, the science and engineering around levees is increasingly under scrutiny at the local and national levels.

Locally, a growing body of research and analyses (e.g., Florsheim and Dettinger 2007, Moss and Eller 2007, Mount and Twiss 2005, Burton and Cutter 2008, Lund et al. 2010, CDWR 2009) has been undertaken to understand the likelihood of, the factors driving, and the impacts of future levee failures in the delta. Some ominous projections have been produced. For example, the U.S. Bureau of Reclamation (USBR) reported in 2008 that “[a] breach of one or more of the central delta levees could result in the temporary or long-term disruption of the water supply for about two-thirds of the state’s residents and for about half of the state’s irrigated agriculture.” In the tricounty area encompassing much of the delta, 1.3 million people are projected to live behind levees by 2020, many of whom are considered socially vulnerable (e.g., infirm and institutionalized, elderly, non-English speakers) (Burton and Cutter 2008). Furthermore, efforts at river engineering and flood control management do not appear to have reduced the frequency of breaches in the Sacramento–San Joaquin system (Florsheim and Dettinger 2007).

The hazards contributing to levee failure are likely to increase in the fu-

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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ture (CDWR 2009). The relationship between small floods (2-3 return-year interval) and levee breaching (Florsheim and Dettinger 2007) may result in increasingly frequent breaching as small floods are projected to increase in frequency in the system with climate change (Dettinger et al. 2006). Sea level rise and ongoing subsidence will further weaken the stability of the levees (Mount and Twiss 2005).

Expectations surrounding an increasingly unstable levee network are documented and include islands filling with water, potential for secondary failures, salinity intrusion, reduction in water quality, channel incision, and suspended water exports (Mount and Twiss 2005, CDWR 2009). Analyses indicate that widespread failure along the levee network, as would occur with a 6.5-magnitude earthquake, would result in up to $40 billion in damages (CALFED 2007). The 2004 levee failure along the Jones Tract alone is estimated to have cost Californians over $100 million (Burton and Cutter 2008).

As politicians, scientists, and engineers look toward more sustainable water management in the delta, the instability and interdependence of levees is likely to be a chokepoint for achieving any measure of water-supply reliability or ecosystem recovery. Continuing the status quo of improving levees, raising highways, and additional protective infrastructure (CDWR 2009), which characterized the 2006 congressional response to concerns over levee instability, will not always be the most environmentally sustainable or economically defensible response in the years ahead. Indeed, researchers (Suddeth et al. 2010) have found that levee upgrade could not be economically justified for the 34 subsided delta islands they examined.

When considering repair of unstable (and breached) levees in the delta, a transparent and vetted prioritization system is needed. The social and economic benefits and costs of repairs of levees (e.g., Suddeth et al. 2010) (see Ohio Levee Classification system, Ohio Emergency Management Agency 2011) should be balanced against those of repairs for islands where subsidence and other factors have reduced the economic and societal value of the land. Such a balancing should not be based solely on economic values. As highlighted during Hurricane Katrina, and as documented in the delta (Burton and Cutter 2008), socially vulnerable citizens tend to cluster within high-risk flood areas. Thus, decisions regarding levees and flood risk management may need to be localized to address differences in culture and language, age, and mobility of those protected by the levees (Burton and Cutter 2008). An approach to prioritizing repair and abandonment of levees should include a mix of economic and social values.

In some cases, managers will need to look beyond levee repair and follow efforts at the national scale that have emphasized revisiting flood management policies and engineering. For example, the U.S. Army Corps of Engineers has adopted flood and levee management strategies, following

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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levee failures during Hurricane Katrina, that include restricting building and repairing levees in areas of high risk (Sills et al. 2008). From a policy perspective, the current vulnerabilities to flooding largely result from policies that foster perceptions that construction of levees eliminates risk (Tobin 1995). Thus, rather than always moving to repair levees and maintain incorrect notions regarding flood control, experts have recommended at the national level that water resources managers “Give full consideration to all possible alternatives for vulnerability reduction, including permanent evacuation of flood-prone areas [and] flood warning” (Interagency Flood-plain Management Review Committee 1994).

Finally, given the dependence of the delta hydrology on the network of levees, the benefits provided by restoration activities will also depend on the status of the levees. Restoration projects should be designed with flexibility to accommodate potential changes in hydrology due to levee failure. For example, constructing wetlands in areas where levees and other infrastructure (e.g., roads, docks) severely constrain the hydrology and resulting habitat types are likely not to maintain their benefit over the long-term as levees fail, sea level rises, and upstream hydrology changes.

WHAT ARE REASONABLE EXPECTATIONS FOR DELTA RESTORATION?

The committee’s statement of task (Appendix C) includes a request to “[a]dvise, based on scientific information and experience elsewhere, what degree of restoration of the Delta system is likely to be attainable, given adequate resources.” There are many uncertainties, including to some degree about the goals of the restoration (NRC 2011), but a few things can be said with confidence.

First, the delta as it was before large-scale alteration by humans (before about 1880) cannot be recovered. We probably cannot even know with precision and detail what the pre-alteration delta looked like. Many of the species in the delta are new (introduced from elsewhere), and even if one could remove all the human-made infrastructure, which is not economically or practically feasible, the biophysical environment would not return to its former state. This is because the changes that already have occurred in response to the human-caused changes in the delta preclude some restoration pathways. Indeed, an earlier NRC committee (NRC 1996) advocated the use of the word rehabilitation instead of restoration and defined it as meaning “a process of human intervention to modify degraded ecosystems and habitats to make it possible for natural processes of reproduction and production to take place. Rehabilitation would protect what remains in an ecosystem context and regenerate natural processes where cost-effective opportunities exist.”

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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Second, as long as the delta is not radically transformed or contaminated or otherwise destroyed, a functioning ecosystem probably will remain there. It will differ from the original and probably from the current ecosystem in its species, habitats, productivity, and other aspects, but it will continue to have algae, invertebrates, fish, birds, and other creatures. It will provide for some recreation and it will continue to provide some ecosystem services. And it, like all ecosystems, will continue to respond to environmental changes. We live in a human-modified world that has created many “novel” ecosystems (Figure 4-4; Hobbs et al. 2009), including the one now found in the delta. Hobbs and colleagues identify two categories of human-induced change: biotic (primarily species introductions and extinctions) and abiotic change (e.g., land use, climate change). Both sources of ecosystem modification are prominent in the delta. The degree of change in both biotic and abiotic categories affects the likelihood of restoration (Figure 4-4). Many species currently in the delta are invasive, and, as in other systems, their elimination is highly unlikely (Vander Zanden and Olden 2008). Moreover, undoing all of the abiotic changes is neither economically desirable nor practically feasible. These two dimensions of

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FIGURE 4-4 Types of ecosystem that develop under varying levels of biotic and abiotic alteration. (a) Three main types of system state: (i) historical, within which ecosystems remain within their historical range of variability; (ii) hybrid, within which ecosystems are modified from their historical state by changing biotic and/ or biotic characteristics; and (iii) novel, within which systems have been potentially irreversibly changed by large modifications to abiotic conditions or biotic composition. (b) The state space can be divided into an area within which restoration to a system within the historic range of variability remains feasible (which includes some or most hybrid systems), areas within which restoration of ecosystem structure and/ or function can be achieved without a return to historic system characteristics, and an area within which restoration is likely to be difficult or impossible and hence alternative management objectives are required.
SOURCE: Hobbs et al. (2009).

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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change preclude recovery of the delta as it was before large-scale alteration by humans (before about 1880). Scott et al. (2005) pointed out that for many species, their survival will depend on continued human inputs. They suggested that “[p]reventing delisted species from again being at risk of extinction may require continuing, species-specific management actions.” They called such species “conservation-reliant.” The committee agrees with their conclusions.

It is common for ecosystems to exhibit nonlinear changes in aspects of their structure and functioning (Scheffer et al. 2001). Abiotic factors (such as climate, e.g., Scheffer and Carpenter 2003), biotic factors (changes in species distributions, e.g., Frisk et al. 2008), or an interaction of both (van Nes et al. 2007) often underlie these abrupt changes. However, while external factors can influence ecosystem conditions in slow and imperceptible ways, they can also trigger a regime change as critical thresholds, or tipping points, are crossed. Importantly for restoration and rehabilitation, the critical thresholds that bring about regime change in one direction may be different from those bringing about change in the reverse direction. As a result, ecosystem change can show hysteresis (Scheffer et al. 2001, Tett et al. 2007); sometimes, reversal is not feasible. The presence of these nonlinear processes suggests that the path by which the delta arrived at its current state, even if well understood, is likely not the same path by which the system will move toward any desired state. Even if it were possible to restore the environmental conditions to a historical baseline, the ecosystem may not return to its former state and additional actions or ecological transitions might be required to achieve some more desirable ecosystem condition (Duarte et al. 2009). Moreover, since the pioneering early work of Watt (1947), many ecosystems also show alternative stable states (e.g., Gargett 1997, Fogarty and Murawski 1998, Chavez et al. 2003, Watson and Estes 2011). The implication of this is that the return of an ecosystem to a former state is unlikely, especially with large, complex systems like the delta. With respect to the delta ecosystem, building habitat or restoring flows does not mean “they will come.” Together the experiences from studies of change in ecosystems around the world suggest the importance of considering both alternate states and hysteresis in visions for a future delta.

However, the presence of substantial biotic and abiotic changes, together with the potential for alternative stable states, does not mean that we cannot effect changes to yield a more desired delta ecosystem. Recently, Choi (2007) has drawn the analogy that, just as a prosthesis rehabilitates a patient by restoring the function of a lost limb and not the structure, we should focus more on restoring ecosystem functioning than individual constituents. Additionally, just as the biophysical environment today is different from that present previously, it is likely that the biophysical environment will continue to experience change, and thus rehabilitation should

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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focus on promoting changes that lead to resilient ecosystems that promote desirable ecosystem services (Harris et al. 2006). A focus on rehabilitation would act to protect the delta ecosystem that remains while promoting the regeneration of natural processes and functions that would lead to a resilient ecosystem that produces services valued by society (e.g., water supply, recreational opportunities, and a sense of place).

A new focus on ecosystem functioning and resilience as rehabilitation targets does not mean that we abandon efforts on restoring individual species, nor does it mean a laissez-faire acceptance of the current degraded ecosystem. Indeed, ensuring ecosystems that are resilient in part relies on maintaining resilience at the individual species level. Maintaining genetic diversity within individual populations increases the likelihood that the population will be sustained in the face of environmental change—a point recognized in recent hatchery and recovery plans for salmon by the National Marine Fisheries Service (NMFS). Managers must promote diversity at the species level and in the configuration of the ecosystem so that it too is resilient to change. However, we should recognize also that we cannot “will” a sustainable ecosystem that contains a list of desired species. We should instead focus on management that promotes diverse, resilient ecosystems that sustain most desired species and that provide the greatest suite of ecosystem services.

Third, and perhaps most important, there appears to be considerable capacity to guide the response of ecosystems to environmental change. The larger and more complicated the ecosystem and the greater the changes caused by humans, the harder it is to produce desired changes, but even severely altered ecosystems can be rehabilitated to varying degrees. No ecosystem as large, as complicated, and as significantly altered as the bay delta has been fully rehabilitated, and indeed “full rehabilitation” seems to be an undefined and possibly unachievable goal. Managers must maintain flexibility in their definition of an achievable target, because no matter what humans do, the system will continue to evolve, both ecologically and genetically. Nonetheless, ongoing efforts in comparable ecosystems such as the Everglades (NRC 2007, 2008b, 2010), the Klamath Basin (NRC 2004, 2008a), the Columbia River, and others have shown limited recovery in some areas. While some of these activities are still in their early stages and all are beset by many challenges, they do provide some cause for optimism for the delta’s future if a sustained, thoughtful, long-term, and well-funded effort is mounted.

Finally, experience in the delta and in other ecosystems highlights the importance of clear, well-articulated goals and of a workable governance system (Chapter 5). While no plan, however well thought out and developed, will be fully realized, without an effective plan, rehabilitation efforts are doomed. The development and implementation of such a plan depend

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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heavily on a workable governance system (see, e.g., recent NRC reviews of the Everglades restoration efforts cited above, and especially Chapter 5 of this report).

CONCLUSIONS

Habitat loss and alterations, climate change, and unpredictable levee failure pose significant challenges in the formulation of sustainable plans for the bay-delta ecosystem. There are many opportunities to steer the future evolution of the ecosystem by addressing future challenges.

Extensive physical changes in the delta ecosystem and the tributary watersheds, and continuously evolving changes such as land subsidence in the delta islands, will not allow the recreation of habitat as it once existed in the predisturbance state. Delta restoration programs will need to balance consideration of an ecosystem approach with the Endangered Species Acts’s (ESA’s) and other factors’ emphasis on individual species (e.g., NRC 1995). Programs will need to focus on the interaction of biological, structural, and physical aspects of habitats and how they may change in the future. Even without ESA-listed species, there still would be a need to guide the ecosystem toward desirable states.

Climate change assessment provides a reasonable picture of what the delta may experience in the future and that picture needs to be incorporated into restoration planning. Such an outlook includes a larger fraction of winter precipitation occurring as rain in tributary watersheds in the Sierra Nevada, reduction in snowpack and correspondingly of water supply during late spring and summer, reduction in water-storage opportunities with a corresponding reduction in the ability to mitigate floods and meet minimum flow targets, challenges in managing the cold-water pools of the upstream reservoirs, and increased probability of water temperatures exceeding lethal limits for delta smelt, salmon, and other species. Many of these changes are already being observed. Projected increases in the mean sea level and the extremes have the potential to increase the frequency of levee failures and inundation of islands, particularly if upstream floods, astronomical tides, and winter storms coincide in the future when the mean sea level has increased due to warming. Sea level rise also has the potential to increase saltwater intrusion and degrade water quality with a significant impact on water exports.

Dealing with climate change implications will require a nonstationary viewpoint that recognizes changes in hydrology, rising sea level, and increased temperature. Planning and evaluation of future scenarios will need to address the uncertainties in projections, integrated analysis, and the development of risk management strategies (e.g., adaptive management). Climate change implications and the continued increase in water demands

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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in the bay-delta system and beyond will exacerbate the competition for water and limit the ability to meet the co-equal goals.

Future planning should include the development of a climate change– based risk model and analysis that incorporates data on the actual changes in delta conditions as well as alternative future scenarios and their probability. The objective should be to develop the basis for priorities for future investments in water-management programs. The real challenge is deciding how to adapt to a new environment. The uncertainties are higher about the environmental aspects of operations than about the reliability aspects of water deliveries. For example, expected environmental and other changes will force policy choices related to replacing water storage currently provided by snow on the ground. Strategies to deal with the expected and unprecedented changes will need to consider many factors, including targeted demand management, increased surface-water and groundwater storage consistent with minimizing environmental impacts, enhanced flexibility in the water-management system through operational optimization and maximum flexibility for moving water, and developing an understanding of and establishing environmental flows for the ecosystem. As described in more detail in Chapter 5, comprehensive strategies would include development of a planning and regulatory framework that incorporates concepts of shared adversity during times of water shortage. They also would include adoption of measures designed to mitigate water temperature increases that are harmful to fish species.

The instability and interdependence of levees are likely to be major issues for achieving any measure of water-supply reliability or ecosystem rehabilitation. Continuing the status quo of improving levees will not always be the most environmentally sustainable or economically defensible response in the years ahead. Indeed, changes in the levee system, and even removal or modification of some levees, could be good for at least parts of the ecosystem. Levee failures are inevitable over the long term and it is essential to plan for either the major investment needed to repair and maintain the levees or the prospect of fundamental change. When considering repair of unstable (and breached) levees in the delta, a transparent and vetted prioritization system is needed. Future delta planning efforts should give full consideration to a wide range of alternatives for vulnerability reduction, including permanent evacuation of flood-prone areas and flood warning. Restoration projects should be designed with flexibility to accommodate potential changes in hydrology due to levee failure.

Resource managers dealing with the delta will need to determine the degree of “restoration” achievable through intervention and adaptation. There is agreement that the delta as it existed before large-scale alteration by humans cannot be re-created. With respect to species, habitats, productivity, and other aspects, the future delta will still be a functioning ecosystem

Suggested Citation:"4 Environmental Change: Challenges and Opportunities." National Research Council. 2012. Sustainable Water and Environmental Management in the California Bay-Delta. Washington, DC: The National Academies Press. doi: 10.17226/13394.
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but different from the one that exists today. Furthermore, ecosystems—even those with minimal human impacts—are not constant in space and time. They evolve. But they can retain salient features for long periods, and despite significant changes in both biotic and abiotic conditions that have occurred during the last 150 years, there is a considerable capacity to guide the direction of the delta toward a more desirable future by focusing on a functioning resilient ecosystem without abandoning individual efforts to protect native species. Our experience with other ecosystems suggests that to achieve success, clear goals and a workable governance system will be needed.

Achieving the above will require extensive, thoughtful, and transparent planning. That planning will need to include finding ways to reconcile diverse interests without pretending that everybody can have what they want. The next chapter considers approaches for such planning, as well as constraints on it.

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Extensively modified over the last century and a half, California's San Francisco Bay Delta Estuary remains biologically diverse and functions as a central element in California's water supply system. Uncertainties about the future, actions taken under the federal Endangered Species Act (ESA) and companion California statues, and lawsuits have led to conflict concerning the timing and amount of water that can be diverted from the Delta for agriculture, municipal, and industrial purposes and concerning how much water is needed to protect the Delta ecosystem and its component species.

Sustainable Water and Environmental Management in the California Bay-Delta focuses on scientific questions, assumptions, and conclusions underlying water-management alternatives and reviews the initial public draft of the Bay Delta Conservation Plan in terms of adequacy of its use of science and adaptive management. In addition, this report identifies the factors that may be contributing to the decline of federally listed species, recommend future water-supple and delivery options that reflect proper consideration of climate change and compatibility with objectives of maintaining a sustainable Bay-Delta ecosystem, advises what degree of restoration of the Delta system is likely to be attainable, and provides metrics that can be used by resource managers to measure progress toward restoration goals.

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