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2. Structure and Functioning of Riparian Areas Across the United States
Pages 49-143

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From page 49...
... Watershed features such the slope of the land, size of the watershed, storage capacity of the soil, and supplies of groundwater and sediment interact with climate to modulate or amplify these effects. Within the riparian area itself, further sources of variation can be found in channel morphology, sediment dynamics, and floodplain structure.
From page 50...
... Headwater networks of very small streams accumulate rainfall, overland flow, snowmelt, or aquifer discharge, sending variable amounts of water downstream to increasingly larger channels. The water budget of all streams and rivers is determined by climate and by other watershed attributes such as topography, soil type, bedrock substrata, groundwater discharge, and vegetation.
From page 51...
... STRUCTURE AND FUNCTIONING OF RIPARIAN AREAS ACROSS THE U.S. 51 full channel, is the discharge of the 1.5- to 3-year return period storm (Dingman, 1984~.
From page 52...
... The size and character of streamside riparian areas is directly related to water delivery to and flux through the watershed. One pattern is the tendency for riparian areas to be expansive next to big, larger-order rivers, which in part reflects multiple hydrologic sources (e.g., seasonal overbank flows from the river, flood-related flows in secondary channels, and groundwater discharge, all discussed in detail in a later section)
From page 53...
... . Another generalization is that where fluvial systems encounter relatively wide valleys and low channel gradients, they typically develop a system of meanders and floodplains that represent both sediment and water discharge regimes in dynamic balance with valley and channel gradients, channel morphology, and riparian vegetation.
From page 54...
... Erosion and Deposition The processes of erosion, transport, and deposition continually disturb and reshape the riparian environment. Materials from upstream sources such as erosion zones along hillslopes and riparian terraces or landslides are sorted by flowing water and transported downstream until the physics and energetics of the transport process dictate deposition either in the channel or on the floodplain of the river.
From page 55...
... These effects may be transmitted in both the upstream direction (backwater effects, including upstream bed material storage and an altered channel morphology; channel incision and gully head cuts) and the downstream direction (higher levels of sediment transport with the potential for increased channel instabilities)
From page 56...
... Thus, streamside riparian areas are responsible for the dissipation of energy associated with flowing water. The flow resistance, or roughness, of a stream reach, caused by the physical configuration of its channel, streambanks, and floodplains as well as by the riparian plant communities, can be described by a roughness coefficient, such as Manning's n (Leopold et al., 1964~.
From page 57...
... At high flow, streambanks, floodplains, and their associated vegetation provide resistance to flowing water, thus locally altering patterns of scour, sediment transport, and deposition (Sedell and Beschta, 1991~. For example, low velocity zones have been observed to develop when floods pass through riparian forests, creating sites for the retention of sediment and organic matter and refuges for aquatic organisms (Swanson et al., 1998~.
From page 58...
... Seasonal dynamics in flow and sediment transport constitute the foundation of riparian structure and thus influence the resulting colonization by riparian species and the many functions performed by these areas. Moisture availability and anoxia in riparian soil are additional factors that closely follow the distribution of grain sizes determined by fluvial processes.
From page 59...
... As a result, flow pathways are seldom entirely parallel or entirely perpendicular to the main channel but instead occur diagonally toward the channel in a downstream direction. The major controls on orientation of groundwater flow paths are hydraulic properties of aquifer materials, regional gradient, and sinuosity of channel (Larkin and Sharp, 1992~.
From page 60...
... Groundwater discharge points tend to occur at the upstream ends of pools, the upstream side of meanders, anywhere along the channel thalweg (deepest area of central channel) , and within side channels or alcoves in streams and rivers (Harvey and Bencala, 1993~.
From page 61...
... The importance of groundwater discharge points along channels is twofold. First, areas along channels that collect groundwater discharge tend to favor establishment of rich npanan vegetation, especially in dry climates where water avail A , | SYRACUSE ~ ~~—~ UPRIVER 0~ i a ~~~ I ~ /~\.
From page 62...
... If comprised of coarse sediments, the riparian area can usually store large quantities of hillslope runoff and release it to the channel by groundwater discharge. Riparian sediments that are relatively fine and are lower in permeability than other soils of the watershed generally cannot store large quantities of water quickly enough, leading to rapid expansion of saturated areas.
From page 63...
... larly topography, sediment hydraulic properties, and antecedent groundwater levels and soil moisture. Weather is also important through its effect on the intensity and duration of precipitation, temperature, and solar radiation patterns.
From page 64...
... Concept dominates \ \~ Shallow subsurface flow dominates; peaks produced by return flow and direct precipitation Arid to sub-humid climate; thin vegetation or disturbed by humans Humid climate; dense vegetation i. Climate, vegetation and land use RIPARIAN AREAS Thin soils; gentle concave footslopes, wide valley bottoms; soils of high to low permeability Steep, straight hillslopes; deep, very permeable spoils; narrow valley bottoms o cd o o Em FIGURE 2-8 Dominant processes of hillslope runoff in response to rainfall.
From page 65...
... C) 2000 by John Wiley and Sons, Inc.
From page 66...
... . Bank storage in npanan areas can affect water storage, chemical transformations in streams and nvers, surface water temperature, and the composition and extent of npanan plant communities.
From page 67...
... flow paths, and (C) hyporheic flows through bends of meandering stream.
From page 68...
... After the peak spring flows in the Cedar River, pesticides were slowly discharged back to the river over a period of weeks to months, accomplishing a dilution of pesticide reaching the river. The other type of bidirectional interaction is hyporheic exchange, which is the temporary routing of water through gravel bars and the alluvium surrounding stream channels (i.e., the hyporheic zone see Figure 1-4~.
From page 69...
... In contrast, mesquite uses a mixture of groundwater and unsaturated zone water depending on tree size, cottonwood uses mostly groundwater, and willow uses only groundwater. When combined with sap-flow measurements, water-stable isotopes determine how water use by riparian trees changes with forest age, groundwater levels, and climatic fluctuations.
From page 70...
... Dissolved inorganic nutrients and nutrients associated with fine particulate organic or inorganic matter move with the flowing water, while nutrients in biotic compartments, such as microbes, periphyton, aquatic plants, and riparian trees, spend much longer in one place within the stream corridor. The cycling of nutrients between transported and fixed components is the basis of the Nutrient Spiraling Concept, which refers to the sequences of movement and temporary retention that occur during downstream transport (Newbold et al., 1982~.
From page 71...
... As shown in Figure 2-13, biogeochem~cal interactions between riparian areas and channels are probFlood Season ~ phase Water level Dry Season Terrestrial phase River-floodplain nutrient transfer Floodplain-river nutrient transfer Rapid growth of aquatic flora Fish Spawning Maximum fish growth Vegetation regeneration Floodplain desiccation FIGURE 2-13 The influence of the flood pulse within the river-floodplain complex. SOURCE: Reprinted, with permission, from Bard and Wilby (1999~.
From page 72...
... Riparian soils (defined topographically as valley bottom areas that tend to become saturated during storms) possess greater soil N concentrations, higher-quality particulate organic carbon (as measured by C:N ratio)
From page 73...
... STRUCTURE AND FUNCTIONING OF RIPARIAN AREAS ACROSS THE U.S. 73 A B Overland Flow "'\' Overland Flow \ Or arll'~ Ados horus / FIGURE 2-14 Fate and transport processes for (A)
From page 74...
... Filtration is more significant in trapping larger soil particles, aggregates, and particulate organic matter, while adsorption to clay and organic matter in soils is more effective in trapping dissolved compounds with positive charges, such as orthophosphorus, heavy metals, and some pesticides. Adsorption of chemicals to the soil surface during overland flow is probably not very significant because of the short contact time and because adsorption sites are likely filled with previously adsorbed molecules (Dillaha et al., 1989~.
From page 75...
... Given their ample organic matter and diverse microbiology, it is not surprising that riparian areas support denitrification (Groffman et al., 1992; Addy et al., 1999) , with riparian forests reported to remove 30-40 kg N ha-i yr-i under suitable conditions (Lowrance et al., 1995~.
From page 76...
... Thus, the extent to which these mechanisms occur is dependent on the amount of time that runoff and associated chemicals are retained in the riparian area, which is in turn largely a function of hydrology. Overall pollutant removal occurs to the greatest extent when overland flow and shallow subsurface flow are distributed uniformly across the riparian area.
From page 77...
... Incoming solar radiation from sunlight influences air temperature, precipitation, and the subsequent apportioning of precipitation into evapotranspiration, subsurface recharge, and watershed runoff. Within the contiguous 48 states, solar radiation exhibits significant seasonal variability.
From page 78...
... Finally, aspect and slope further influence the amount of energy available at a particular location: during clear sky conditions, south-facing slopes receive more incoming solar radiation per unit area than do north-facing slopes (Reifsnyder and Lull, 1965~. Annual precipitation is highest in the eastern United States, particularly along the Gulf of Mexico coast, with up to 155 cm in Tallahassee, Florida (Figure 2-16~.
From page 79...
... Even in tectonically stable areas, however, the primary influences on the structure of riparian plant communities are fluvial processes in particular, floods and the associated transport of sediment within streams. Floods and overbank flows occur when stream discharge exceeds channel capacity.
From page 80...
... These factors determine the extent of infiltration versus overland flow, the proportion of soil water that is evaporated, the flow paths of precipitation across or through soils, and the proportion of soil water that is recharged to groundwater. Together, they affect the timing and rate of water delivery to channels, as well as the total runoff.
From page 81...
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From page 82...
... Summer thunderstorms are dominant contributors to runoff in western mountains, the arid Southwest, and the Great Plains because winter precipitation is often low and because sparsely vegetated surfaces contribute a greater percentage of precipitation to overland flow. Together, these climatic factors determine patterns of peak runoff that become progressively later in the year from east to west across the United States.
From page 83...
... C) 1995 by John Wiley & Sons, Inc.
From page 84...
... In contrast, if the soils are permeable, overland flow may be limited but subsurface flow and groundwater recharge will be high. The key variables used are the distribution of landscape relief (maximum minus minimum elevation)
From page 85...
... Geologic uplift that increases or decreases channel slope can cause, respectively, greater channel incision or greater overbank flow frequencies (Burnett and Schumm, 1983~. Even in tectonically stable areas, however, the primary influences on the structure of riparian plant communities are fluvial processes in particular, floods and the associated transport of sediment within streams.
From page 86...
... Flood peak Base Future years: Normal peaks PeakGerminatTon r//~/~fl/~/'l/l Flooding Drought mortality Amorality Pioneer Recruitment on Plood Deposition Surfaces FIGURE 2-20 Hydrogeomorphic control of recruitment of woody pioneer species. Seed germination, early seedling mortality, and tree recruitment are shown in relation to annual high and low flow lines along four bottomland cross sections (A-D)
From page 87...
... (A) Cross section of typical bottomlands in the low-gradient streams of the Coastal Plain of the southeastern United States.
From page 88...
... Such replacement tends to reduce the complexity of riparian forests with predictable consequences for nesting birds, shade-adapted plants, and other tree-dependent organisms. Change in relative sea level is yet another, albeit more chronic, natural disturbance that causes changes in coastal riparian areas.
From page 89...
... Moisture Availability and Anoxia Although physical disturbances are important in higher-order streams with strong flows and in headwater streams with steep gradients, soil moisture and
From page 90...
... The different gradients of plant species richness brought about by moisture availability are illustrated in Figure 2-23. In this case, the number of species found within the riparian area of arid Sierra Nevada streams is markedly higher than in adjacent uplands, while species richness throughout the more humid Cascades riparian areas and uplands is relatively uniform (although species composition is not identical)
From page 91...
... moisture availability, and the frequency, duration, and intensity of floods are selective factors that help shape the spatial patterns and species composition of riparian plant communities. The dynamic nature of many stream channels leads to cyclic changes in vegetation as the result of floodplain erosion and point bar deposition.
From page 92...
... The diversity in riparian vegetation stems not so much from species richness within individual plant communities but rather from the existence of broad plant community types as well as age and structural diversity within plant communities. This large-scale heterogeneity is a result of the extreme conditions of disturbance, wetness and dryness, and other fluvial processes characteristic of riparian areas.
From page 93...
... Alder also colonizes bare soil, but is eventually replaced by a continuous deciduous canopy of balsam poplar in the absence of further disturbances. Next, productive stands of white spruce develop on the rich alluvial soils, but eventually organic matter and a moss ground cover accumulate.
From page 94...
... Pacific Northwest and Coastal Mountains The Pacific Northwest and coastal mountains support a diverse woody plant flora with a high percentage of endemic species, particularly in the California Floristic Province. The species dominating riparian forests in California are related to air temperature, groundwater depth and aeration, and frequency and intensity of disturbance (Holstein, 1984~.
From page 95...
... In the northwestern regions, riparian forests included black cottonwood, willow, hawthorn, water birch, chokecherry, and gray alder. In the southern Great Basin, Fremont cottonwood grows in association with several willow species and mesquite.
From page 96...
... In the northern Rockies of Montana, black cottonwood is the dominant riparian tree. Other riparian trees or shrubs in the Rocky Mountains of regional importance include serviceberry, chokecherry, hackberry, and hawthorn.
From page 97...
... For instance, the riparian forests of eastern Oklahoma support a rich tree flora, including black birch, sweet gum, water tupelo, American sycamore, bald cypress, hackberry, honey locust, elm, hickory, and boxelder (Bruner, 1931~. In the northern Great Plains, the diversity of the riparian tree community is also greater than in the western plains.
From page 98...
... Along streams with steeper flow gradients and higher water velocities, the woody vegetation is dominated by hazel alder, silky dogwood, American witch hazel, possum hew, and black willow. On the narrow floodplains along streams in the western Appalachian Mountains, riparian forests are dominated by river birch or a more diverse association of bitternut hickory, red maple, tulip tree, black cherry, and black locust (Wolfe and Pittillo, 1977~.
From page 99...
... found that the composition of riparian woody plant communities was similar to those found along the higher-gradient Passage Creek. However, the riparian areas of lowergradient rivers included additional species sweetgum, water hickory, silver maple, American beech, tulip tree, bald cypress, and water tupelo.
From page 100...
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From page 101...
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From page 102...
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From page 103...
... It has been well established that riparian plant canopies particularly those of forest vegetation are effective interceptors of incoming solar radiation and can thus greatly reduce the amount of solar energy available to a stream or river (Brown, 1969~. The shading effects of riparian plant canopies can prevent or retard the rate of stream warming during clear-sky conditions in summer months when streamflows are often low and high instream temperatures are of concern to aquatic biota (especially cold-water fish)
From page 104...
... Furthermore, channel morphology can play an important role in the connectivity of instream flows with the hyporheic zone and groundwater. For example, alterations to riparian plant communities that contribute to channel incision may sufficiently modify hyporheic and groundwater connectivity such that thermal regimes and low-flow periods are significantly altered.
From page 105...
... Several processes can recruit trees from riparian forests into adjacent bodies of water, including individual tree mortality, blowdown, bank erosion, and, in steep mountainous terrain, landslides. Large-wood accumulation in streams and rivers has historically been an important feature of forested riparian systems for all
From page 106...
... Large wood creates roughness elements in flowing water, which shape pools and riffles, create variable velocities, and increase the residence time of water (Sedell and Beschta, 1991~. Large wood helps streams and rivers slow the downstream routing of sediment and organic matter by providing increased hydraulic resistance to flow and encouraging the local deposition of these materials.
From page 107...
... In floodplain rivers, high flows can move even the largest logs, and the extremely wide channels provide little resistance to movement. Wood outside the channel dissipates the power of overbank flows, traps sediment during flooding, serves as wildlife habitat and refuge, provides nursery logs for certain tree species that typically regenerate on downed wood, and supports microbial processes and transformation of critical nutrients (Maser et al., 1988~.
From page 108...
... The establishment, growth, and succession of individual plants and groups of plants represent a mechanism by which riparian plant communities can continually respond to natural changes in streamflow and sediment loads and to local adjustments in channel morphology (Kalliola and Puhakka, 1988; Kalliola et al., l991~. In addition to the immediate effects of resisting fluvial erosion of streambanks, vegetation plays a similarly important role with regard to floodplain development.
From page 109...
... In the arid Southwest, 70 percent of threatened and endangered vertebrate species are listed as riparian obligates (Johnson, 1989~; 60 percent of all vertebrate species are so defined (Ohmart and Anderson, 1982~. Even in the relatively mesic environment of Wisconsin, 80 percent of plant and animal species on the state's endangered list live all or part of their lives in the riparian area of lakes (Korth and Cunningham, 1999~.
From page 110...
... The structural diversity of plant species in riparian areas creates a wide variety of feeding niches for herbivores and carnivores alike. Various types of dispersal occur in riparian areas, including immigration, emigration, and migration.
From page 111...
... As forested streams increase in size from headwaters to large rivers, instream primary production contributes more to the food base of stream ecosystems, although floodplains and their forests still deliver large portions of the energy supply to large lowland rivers (Vannote et al., 1980; Minshall, 1988; Junk et al., 1989~. Riparian forests with multiple canopy layers, species, and age classes offer a wider array of food resources and physical habitats than do simple uniform plant communities (Gregory et al., 1991; Swanson et al., 1992; Osborne and Kovacic, 1993; Moyle and Yoshiama,1994~.
From page 112...
... The wood turtle of the northeastern United States uses a variety of riparian habitats lying within about 360 meters of its home river for foraging, basking, and nesting (Vogt, 1981; Ewert, 1985~. Other species of turtles, such as the snapping turtle, are almost exclusively aquatic but need the riparian area for nesting (Harding, 1997~.
From page 113...
... Birds The importance of riparian areas as breeding habitat for birds is well known to birdwatchers and professional ornithologists alike. Because riparian areas are inherently diverse in plant species and varied in vertical and horizontal structure, they provide a variety of niches for birds, as documented by numerous studies (MacArthur, 1964; James, 1971; Karr and Roth, 1971; Whitmore, 1975; Rice et al., 1983, 1984~.
From page 114...
... Even in mesic forests of Canada, boreal riparian conifer stands have higher avian diversity and abundance than do the adjacent coniferous uplands (Larue et al., 1995~. In some landscapes lacking contrast between riparian and upland vegetation, such as in parts of the Pacific Northwest, avian diversity in riparian areas has not been found to be significantly greater than in uplands, although some differences in community composition have been documented (McGarigal and McComb, 1992; Murray and Stauffer, 1995~.
From page 115...
... As illustrated in Box 2-2, riparian areas in the sagebrush steppe have been found to be critically important to sage grouse species as brood-rearing habitat. Work along the lower Colorado River valley has highlighted the importance of riparian vegetative structure to wintering avian species, an often-overlooked habitat component (Anderson and Ohmart, 1977~.
From page 120...
... Rare and relic plant species found only along the Great Lakes shoreline, in addition to the many animal species using riparian areas in this region, are described in Box 2-4. ENVIRONMENTAL SERVICES OF RIPARIAN AREAS The biosphere is often called the human "life support" system because of its paramount importance to maintaining the atmosphere, oceans, and land resources that support human societies.
From page 122...
... (This approach has been used to interpret the societal values of the functions of other ecosystem types [Christensen et al., 19961~. For example, functions related to hydrology and sediment dynamics include storage of surface water and sediment, which reduces damage from floodwaters downstream from the riparian area.
From page 123...
... Riparian areas cannot be thought of in isolation from stream channels. The characteristic geomorphology, plant communities, and associated aquatic and wildlife species of riparian systems are intrinsically linked to the role of water as both an agent of disturbance and a critical requirement of biota.
From page 124...
... 124 TABLE 2-3 Functions of Riparian Areas and Their Relationship to Environmental Servicesa RIPARIAN AREAS Examples of Functions Indicators that Functions Exist On-site or Hydrology and Sediment Dynamics Stores surface water over the short term Maintains a high water table Accumulates and transports sediments Biogeochemistry and Nutrient Cycling Produces organic carbon Contributes to overall biodiversity Cycles and accumulates chemical constituents Sequesters carbon in soil Habitat and Food Web Maintenance Floodplain connected to stream channel Presence of flood-tolerant and droughtintolerant plant species Riffle-pool sequences, point bars, and other features A balanced biotic community High species richness of plants and animals Good chemical and biotic indicators Organic-rich soils Attenuate Maintains climate Contribut Provides ~ terrestri Provides Intercepts Contribute sequest atmosp] Maintains streamside vegetation Presence of shade-producing forest canopy Provides Supports characteristic terrestrial vertebrate populations Supports characteristic aquatic vertebrate populations Appropriate species having access to .
From page 125...
... STRUCTURE AND FUNCTIONING OF RIPARIAN AREAS ACROSS THE U.S. 125 On-site or off-site Effects of Functions Goods and Services Valued by Society cannel ght~nd other I animals canopy to ance Attenuates downstream flood peaks Maintains vegetation structure in arid climates Contributes to fluvial geomorphology Provides energy to maintain aquatic and terrestrial food webs Provides reservoirs for genetic diversity Intercepts nutrients and toxicants from runoff Contributes to nutrient retention and to sequestration of carbon dioxide from the atmosphere Provides shade to stream during warm season Allows daily movements to annual migrations Allows migratory fish to complete life cycles Reduces damage from floodwaters (Daily, 1997)
From page 126...
... In humid regions of the country, vegetation is abundant in both uplands and riparian areas, with species tolerant to high soil moisture and anoxia being more prevalent in riparian areas. In arid regions, plants are often concentrated along the band of adequate soil moisture provided by riparian areas.
From page 127...
... 1985. Dimensions of riparian buffer strips required to maintain trout habitat in southern Ontario streams.
From page 128...
... Water Resources Research 31: 2319-2339. Boose, A
From page 129...
... 2000. Flood frequencies required to sustain riparian plant communities in the Upper Klamath Basin, Oregon.
From page 130...
... Water Resources Research 6: 1296-1311.
From page 131...
... St. Paul, MN: USDA Forest Service North Central Experiment Station.
From page 132...
... 1992. Nitrate dynamics in riparian forests: microbial studies.
From page 133...
... 1984. California riparian forests: deciduous islands in an evergreen sea.
From page 134...
... : II. Testing of the water quality and nutrient cycling component for a Coastal Plain riparian system.
From page 135...
... 2000. Water table dynamics and soil texture of three riparian plant communities.
From page 136...
... Lowrance et al., 1995. Water quality functions of riparian forest buffer systems in the Chesapeake Bay watershed.
From page 137...
... 1995. Nongame bird use of habitat in central Appalachian riparian forests.
From page 138...
... . Minneapolis, MN: American Water Resources Association.
From page 139...
... 1998. Plant species richness in riparian wetlandsa test of biodiversity theory.
From page 140...
... 1988. Plant migration: the dynamics of geographic patterning in seed plant species.
From page 141...
... 1997. Stream channel erosion and change resulting from riparian forests.
From page 142...
... 1998. Role of submersed macrophytes for the microbial community and dynamics of dissolved organic carbon in aquatic ecosystems.
From page 143...
... Journal of the American Water Resources Association 37(2)


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