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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />II <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Currently, riparian vegetation along the Arkansas River in the study reach is largely <br />restricted to the floodway. Fairly dense sandbar willow occupies the immediate riverbank and <br />mature cottonwoods form a continuous, though sparse band throughout the floodway. A few <br />relatively dense stands of younger (4- to 6-inch diameter) cottonwood are present. Often, within <br />about 60 feet of the channel, the understory consists of mixed willow, salt-cedar, grasses, and <br />(especially in autumn of 1998) sunflower. Further from the channel, salt-cedar occurs either <br />scattered throughout grassland areas or in dense monotypic stands along abandoned meanders. <br />Kochia grows in very dense strips along the landward edge of riparian areas in this reach and, in <br />fact, throughout the entire study area. <br /> <br />3.3.3 Functions of Riparian Vegetation <br /> <br />The following discussion highlights the major functions of riparian vegetation and is not <br />intended to be an exhaustive summary. For concise reviews of riparian functions and values see <br />Brinson et al. 1981, Minshall et al. 1989, Davis et al. 1996, and Minckley 1997. <br /> <br />Bank Stabilization. Channel width, depth, and slope are determined to a large degree by <br />bank stability. Vegetation stabilizes banks by directly reducing flow velocities and thus the <br />erosive forces at the soil-water interface (Davis et al. 1996). Roots and rhizomes of bank <br />vegetation bind soil material, increase cohesiveness, and reduce weakening and loosening <br />processes that are often the precursors of entrainment (Thome 1990). The numerous fine roots <br />of sedges and grasses provide greater binding strength than coarse roots of woody plants. A <br />mixture of vegetation is generally preferred since the deeper rooting depth of trees and shrubs <br />provide additional protection to tall banks. Vegetated banks also are drier than unvegetated <br />slopes because soil water is removed by transpiration, effectively reducing the likelihood of mass <br />failure. The net effect of these contributing forces is generally positive; however, in reaches <br />such as the Arkansas River, dense bank vegetation (i.e., saIt-cedar) can exacerbate channel <br />incision and narrowing. <br /> <br />Resistance to flow. Vegetation within the floodway presents an obstruction to water flow <br />that tends to decrease flow velocities. Soil erosion is reduced in vegetated overbank areas and <br />deposition of suspended sediment is enhanced. The magnitude of these effects depends upon the <br />density and type of vegetation. Grasses and short herbaceous groundcover are flattened against <br />the ground surface by flows and present relatively little resistance to flow. Shrubs provide <br />higher resistance due to the stiff, less flexible branches and, if present, large leaf area. Dense <br />mature trees have large cross-sectional areas and can withstand relatively high flows without <br />breaking, therefore, provide the greatest resistance to flow (Vogel 1984). Sparsely distributed <br />trees can actually generate bank scour by accelerating flow around their trunks. <br /> <br />Flood attenuation. Flood discharge is strongly influenced by floodplain area. A larger <br />floodplain will have a lower peak stage than a smaller one for a given discharge. Additionally, <br />larger channel capacity allows a given flood event to pass more quickly. <br /> <br />Sediment load. Riparian vegetation affects stream morphology by regulating sediment <br />supply and points of deposition. As stated previously, overbank vegetation influences sediment <br /> <br />41 <br />