Riparian Areas Functions and Strategies for Management (2002) / Chapter Skim
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3. Human Alterations of Riparian Areas
3. Human Alterations of Riparian Areas
Pages 144-224
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From page 144... ...
Although degradation of native riparian plant communities by forestry, agriculture, and grazing can often be reversed, other practices such as drainage modifications and structural developments in urban areas generally lead to irreversible changes in riparian areas over long time periods.
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From page 145... ...
Similarly, the Willamette River in Oregon has a reduced frequency of overbank flows, disconnected side channels, and greatly reduced potential for maintaining riparian and floodplain forests because of extensive bank stabilization and dam construction (Figure 3-1~. Box 3-1 gives an example of the effects of various hydrologic manipulations on riparian plant communities and ecosystem processes in the arid Southwest.
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From page 146... ...
twentieth century an extremely short time period compared to the many thousands of years over which riparian plant communities have adapted to shifting climatic regimes, runoff patterns, and adjustments in channel morphology. There are currently 75,000 dams on the streams and rivers of the United States (Meyer, 1996; Graf, 1999)
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From page 148... ...
Dams that are used only for flood control and hydropower generation may not significantly diminish the amount of water available to downstream channels, although these structures can have a major effect on the overall flow regime (the frequency, magnitude, and temporal distribution of flows)
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From page 149... ...
. Structures that divert significant volumes of flow reduce the amount of water available to downstream riparian plant communities.
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From page 150... ...
For example, Rood and Mahoney (1990) found that dams contribute to the loss of riparian forests by reducing downstream flows or by altering flow patterns to attenuate spring flooding or stabilize summer flows.
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From page 151... ...
(1985) Rio Grande New Mexico P
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From page 153... ...
In addition, because many bank structures reduce the hydraulic roughness (i.e., the frictional resistance to flow) along the channel margins, flow velocities are greater along the bank during high flows, which often precludes the survival of many riparian plant species.
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From page 154... ...
Avifaunal studies along the Colorado River showed that, on average, the number of species inhabiting a riprapped riparian area was only about half that of an undisturbed river with intact riparian vegetation (Ohmart and Anderson, 1978~. In some cases, the use of rip-rap can have a deleterious effect on water quality.
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From page 155... ...
Locally, the kinetic energy of water flow is concentrated in the stream channel rather than being dissipated across the floodplain during normal overbank flows. In the absence of streambank stabilization, the channelized reach may undergo a period of accelerated erosion that can lead to additional channel incision or channel widening, or both.
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From page 156... ...
156 RIPARIAN AREAS Stage 1: Premodified Stage 2: Constructed Stage 3: Degradation Stage 4: Degradation and · , ~ w~aen~ng \ ~ ,~ ~ ~ i,/ ~ ., Stage 5: Ag3radation and 71:~ Stage 6 Water ~ Slumped material Direction of bed or bank movement Accreted material FIGURE 3-3 Stages of stream and floodplain evolution following channelization that occurred in western Tennessee streams around the 1900s. Sites depicted are just upstream from the channelization.
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From page 157... ...
Setback levees generally allow for natural riparian plant communities and normal floodplain dynamics by maintaining relatively frequent overbank flows, providing detention storage of flood water, and allowing for deposition of fine sediments along the entire streambank and at least a portion of the floodplain. In essence, setback levees represent a compromise between the development goals of protecting floodplain areas from overbank flows and the ecological goals of maintaining riparian and floodplain functions.
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From page 158... ...
For example, sustained declines in the water table of greater than one meter are likely to cause leaf desiccation in cottonwoods, leading to branch die-back and eventual mortality for a significant proportion of the population (Scott et al., 1993~. Groundwater pumping for water supply in the West has caused a decline in the number of miles of river with perennial streamflow that can most easily support healthy riparian forests (Luckey et al., 1988~.
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From page 159... ...
. Second, the water table must be high enough for riparian plants to reach it, or their removal will have no effect on water availability.
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From page 160... ...
(370-1770) To Downstream Users 167 (93-24 Riparian Evapotranspiration Riparian Evapotranspiration, Irrigated Agriculture, and Open Water Evaporation Elephant Butte Reservoir Evaporation FIGURE 3-4 The Middle Rio Grande water budget with units of 106 ma yr-1 .
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From page 161... ...
On the one hand, saltcedar is an exotic whose removal not only could increase floodwater conveyance (although this has yet to be demonstrated) , but could also provide more habitat for native plant species.
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From page 163... ...
The altered hydrology characteristic of row-crop agriculture and some erosion control structures tends to concentrate overland flow within fields and transport it downslope in grass waterways or other ephemeral drainageways (Schultz et al., 2000~. Although grass waterways are very effective in reducing gully erosion, transformation processes that could improve water quality are limited because the upland runoff enters the riparian area as concentrated flow.
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From page 164... ...
They stabilize stream channels and they promote the infiltration of overland flow. They increase groundwater resources by enhancing groundwater recharge in losing streams.
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From page 165... ...
and the Mississippi Delta (5 million hectares) also have significant areas of drained Cropland Because drainage was traditionally a tool for managing soil moisture, the resulting water quality of receiving streams and other ecological factors were
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From page 166... ...
Depending upon conditions such as antecedent soil moisture and storm intensity surface and subsurface drainage were found to increase peak outflow rates by four and two times, respectively (Figure 3-7~. This increased outflow often results in streambank erosion, channel incision, flooding, or other impacts.
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From page 167... ...
As shown schematically in Figure 3-8, subsurface drainage can expedite direct transport of chemicals (such as NO3-N) from the soil zone to surface waters often completely circumventing riparian areas.
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From page 168... ...
as described in Box 3-2. Grazing Domestic Livestock The history of grazing by domestic livestock in much of the world has been one of large-scale degradation of native plant communities (Chancy et al., 1990; Kauffman and Pyke, 2001~.
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From page 169... ...
Secondary effects include
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From page 171... ...
, accelerated erosion, altered competitive relationships among organisms, and changes in plant or animal reproductive success and/or establishment of plants. Long-term cumulative effects of domestic livestock grazing involve changes in the structure, composition, and productivity of plant and animal communities at community, ecosystem, and landscape scales.
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From page 172... ...
and is typically repeated on an annual basis. Characteristics of the riparian plant communities, such as composition, cover, density, or other measures of plant communities, are likely to show significant changes relative to ungrazed areas (Kauffman and Pyke, 2001~.
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From page 173... ...
concluded that "watershed and water quality would improve to their maximum potential" if livestock were removed entirely from federal lands (BLM and USES, 1994~. The USES concluded that livestock grazing is the fourth major cause of species endangerment nationwide, the second major cause of plant endangerment, and the number one cause of species endangerment in certain arid regions of the West, such as the Colorado Plateau and Arizona Basin (Flasher et al., 1994~.
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From page 174... ...
Forestry The removal of trees by forestry operations has the potential to alter longterm composition and character of riparian forests, and thus the structure and function of these systems (Ralph et al., 1994~. If selection harvest methods are
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From page 175... ...
HUMAN ALTERATIONS OF RIPARIAN AREAS 175 employed and small amounts of timber removed, and if the frequency of harvest is separated by several decades, the effects on riparian plant communities may be relatively small. However, where large portions of the standing timber are harvested or where the period between harvest operations is short, substantial changes to the composition, structure, and function of riparian forests almost certainly will result.
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From page 176... ...
Harvest of streamside forests also removes the vegetative cover that can slow the delivery of sediment into streams and retain nutrients, such as nitrogen and phosphorus. As discussed in Chapter 2, riparian forests collectively provide for an array of sustainable processes and functions that make them exceptionally important for maintaining productive aquatic and terrestrial ecosystems (Johnson et al., 1985; USES, 1993; Laursen, 1996; Verry et al., 2000~.
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From page 177... ...
Streamside buffers are generally not designed to mirror the stand composition and dynamics of desired healthy riparian forests for a given age class, especially when harvest decisions are strongly governed by social concerns about economic impacts. Nearly 136 million acres of the nation's forestland are in the public domain, with 85 million acres being managed by the USFS, 8 million by BLM, and 43 million by state, county, and municipal governments.
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From page 178... ...
When a mining operation exposes large areas of bare ground, substantial increases in overland flow and sediment production can occur during rainfall periods. Unless a well-designed and operated system of detention ponds is in place, such runoff may greatly increase sediment loading to nearby streams and rivers.
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From page 179... ...
In these situations, bar-scalping and streambed excavation can greatly influence long-term sediment transport, channel morphology, and bank stability of specific stream reaches. If large amounts of gravel are removed, channel down-cutting or incision may occur, potentially influencing local groundwater levels, the frequency of overbank flows, bank stability, and the character of riparian vegetation (Collins and Dunne, 1989; Kondolf, 1995~.
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From page 180... ...
Although there has been little systematic study of the effects of these snagging activities upon channel characteristics, riparian functions, and floodplain processes, the effects are likely to have been significant. In the early years of this country, transportation of logs and timber to market was a major challenge.
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From page 181... ...
In urban areas, roads and other impervious surfaces can increase peak overland flows, thus fundamentally altering the hydrologic disturbance regime for those systems. Roads can also concentrate overland flows to specific locations where channel erosion and Sullying and accelerated sediment loading may be initiated.
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From page 182... ...
Typically, only a small portion of the precipitation ends up as direct overland flow. Thus, peak flows are moderated by high infiltration rates, and many streams are perennial due to groundwater flow during periods of the year when overland flow is uncommon.
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From page 183... ...
The changing land use and hydrology of urbanizing watersheds have multiple impacts on stream channels, aquatic ecosystems, and water quality within riparian areas. As runoff frequency, volumes, and peak flow rates increase during urbanization, stream channels respond by increasing their cross-sectional area to accommodate the higher flows either through widening of the stream channel, downcutting of the streambed, or both.
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From page 184... ...
A secondary effect of urbanization is caused by changes in how overland flow and shallow subsurface flow enter and transverse riparian areas that remain after development. Prior to urbanization, overland flow enters the more extensive riparian areas as either sheet flow from areas immediately adjacent to the riparian areas or through small ephemeral drainageways, thus allowing sediment to be deposited and other substances to be transformed.
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From page 185... ...
, cluster and other green development approaches can promote properly functioning riparian areas and the environmental services they provide. (Appropriate BMPs might include infiltration systems, detention ponds, minimization of impervious surfaces, and dispersion of concentrated flow from the high-density areas into the green areas.)
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From page 186... ...
and other visitors more frequently and from a wider area than other types of parks and open space (Green and Tunstall, 1992; Cole, 1993~. Boat landings, fishing access points, portages, parks, golf courses, campgrounds, and trails are all recreational enhancements commonly placed within riparian areas, usually without a careful assessment of their potential impacts.
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From page 187... ...
Boat landings are frequent dispersal nodes for unwanted exotic plant species such as purple loosestrife and Eurasian milfoil in the eastern United States, as evidenced by natural resource agency signage at such locations. Golf course construction and maintenance can impact first- and second-order streams that flow through them and their adjacent riparian areas via the removal of natural vegetation and increased loadings of pesticides and fertilizers.
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From page 190... ...
, which has replaced cottonwood and other native riparian plants throughout much of the southwestern United States. Invasion by saltcedar and its subsequent competition with native species is exacerbated by a reduction in flood flows caused by dams and by the lowering of water tables caused by water withdrawal and consumption.
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From page 191... ...
Currently, there are no long-term monitoring systems for tracking the extent of intact riparian plant communities, the composition of riparian communities, or the distribution of exotic species. As a result, it is unlikely that riparian areas will be adequately protected.
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From page 192... ...
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From page 193... ...
With a warmer climate, streamflows in snowmelt systems would decline earlier in the summer and corresponding water tables would drop, with consequences for invertebrates and fish in addition to the lack of a high water table for riparian plants (Scott et al., 1999; Stromberg et al., 1991~. A transition to lower flows under drier conditions would be particularly stressful for the aquatic and riparian areas that are already water-limited (Grimm and Fisher, 1989~.
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From page 196... ...
In contrast, lower water levels would require that plants establish further toward the lake, but this could happen only if protective barrier beaches form along the shoreline to reduce wave energy (Kowalski and Wilcox, 1999~. This situation may become an issue for the Great Lakes in particular, where lake levels are expected to drop rapidly over the next several decades (Chao, 1999~.
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From page 197... ...
Mangroves would be expected at higher latitudes as the frequency of frost decreases. In each case, the composition of riparian plants and animals would be determined by additions of species that migrate to their correspondingly more favorable climatic conditions elsewhere and subtractions of species that become locally extinct because of less favorable environmental conditions.
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From page 198... ...
Recently introduced, non-native species, such as saltcedar, Russian olive, and Chinese privet, may provide insight into traits necessary for the dispersal of native species. These invaders may be expected, however, to also interfere with the gradual spread of indigenous riparian plants because exotics may be able to monopolize available space and nutrients before indigenous plants can arrive (Galatowitsch et al., 1999~.
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From page 199... ...
Environmental Protection Agency (EPA) estimates of current riparian acreage which assume that the riparian area extends 50 ft from the edge of waterbodies are 62 million and 38 million acres, respectively (excluding Alaska)
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From page 200... ...
A more limited source of information on riparian acreage is public land data, which identify 23 million acres of combined riparian areas and wetlands on BLM-administered lands in the United States, although it is not known what portion is riparian (BLM, 1998~.
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From page 201... ...
, stated that "over 90 percent of the native riparian areas along our major desert watercourses have been lost, altered, or degraded." Similarly dire statistics exist for California, where only 5 percent to 10 percent of the original riparian habitat remains (Mac et al., 1998~. A recent report card on Oregon's riparian areas found that riparian forests along the Willamette River have been reduced by more than 85 percent since the 1850s (Gregory, 2000~.
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From page 202... ...
Land cover/ land use information, gathered systematically at regular intervals, can inform many community-based decisions, especially those concerning riparian areas. Fundamental goals for human activity preservation of agricultural soils, preservation of wetlands, control of rural residential sprawl, limitation of impervious surfaces in urban areas can be assessed and quantified through patterns of vegetation derived from remotely sensed data.
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From page 203... ...
HUMAN ALTERATIONS OF RIPARIAN AREAS TABLE 3-4 Wetland Loss By State From the 1780s to the 1980s 203 State Total surface 1780 Wetlands 1980 Wetlands % Surface area that % Wetlands area (acres) estimates (acres)
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From page 204... ...
How Land Use/Land Cover Data Might Be Used Riparian areas and land cover can be mapped using remotely sensed data from different sources (e.g., satellites, high- and low-elevation aircraft, and balloons) and that provide different spectral information (e.g., multispectral, infrared, visible, ultraviolet, videography, or digital photographs)
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From page 205... ...
Although a growing array of remotely sensed information offers many options for assessment of riparian conditions, land use/land cover data for large basins or regions are most efficiently obtained by the processing and analysis of remotely sensed images acquired by satellite. The USGS through the EROS Data Center allows federal government and affiliated users to access Landsat data from both the Thematic Mapper (TM)
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From page 206... ...
are such that there may have been undersampling. This study and other studies show that land use/land cover alone cannot act as an indicator of water quality; quantities that describe the mechanisms by which the chemical inputs to streams are linked to land use must also be considered.
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From page 207... ...
These finer resolution sources could be used by states and local resource management agencies to augment a broader national assessment and provide local detail on spatial extent and composition of riparian vegetation. FWS should help develop a uniform national program for mapping riparian areas that relies on measurements of land cover and land use from broadly available remotely sensed data, such as satellite multispectral data.
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From page 210... ...
Such a program should incorporate broadly available remotely sensed data, such as satellite multispectral data, which could be used to classify and map land cover and land use information in each of the states.
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From page 211... ...
1976. A land use and land cover classification system for use with remote sensor data.
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From page 212... ...
1993. Management impacts on water quality of forests and rangelands.
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From page 213... ...
. Portland, OR: USDA Forest Service, Pacific Northwest Forest and Range Experiment Station.
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From page 214... ...
1991. A field experiment to evaluate the water quality impacts of agricultural drainage and production practices.
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From page 215... ...
1995. Controlled versus conventional drainage effects on water quality.
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From page 216... ...
1998. Lakescaping for wildlife and water quality.
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From page 217... ...
1984. Livestock impacts on riparian ecosystems and streamside managementimplications.
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From page 218... ...
Pp. 305-329 In: Agricultural management and water quality.
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From page 219... ...
2001. Land cover mapping in an agricultural setting using multiseasonal Thematic Mapper data.
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From page 220... ...
General Technical Report INT-95. Ogden, UT: USDA Forest Service.
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From page 221... ...
General Technical Report RMGTR-272. Fort Collins, CO: USDA Forest Service.
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From page 222... ...
1998. Riparian zones as havens for exotic plant species in the central grasslands.
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From page 223... ...
1995. Agricultural drainage effects on water quality in Southeastern U.S.
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From page 224... ...
1998. Agricultural drainage: water quality impacts and subsurface drainage studies in the midwest.
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