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vegetation, recharge and rinse alluvium forming the banks and floodplain, and they enhance <br />productivity and fitness of existing vegetation. <br />Low flows <br />Low flows may be important in certain settings. For example, plants may be reliant upon shallow <br />water tables (that are connected to streamflow) for maintenance and growth. Although some <br />plants are able to deal with lowered water tables by utilizing moisture in the soil, plants obtaining <br />most of their water from water table or capillary fringe (phreatophytes) may become water <br />stressed and suffer from lowered water tables associated with stream dewatering. If atmospheric <br />demands for water are higher than water available to roots, plants may respond by closing <br />stomates (pores for gas exchange on leaves) and reducing transpiration. If the water deficit <br />persists plants may become water stressed and sacrifice leaves or branches (Tyree et al. 1994). <br />kltense or prolonged dewatering of streams can lead to desiccation and death (Rood et al. 2003). <br />If water tables slope toward the stream, reduced low flows may steepen the water table surface <br />and increase the depth to water across the floodplain, but dewatering a gaining stream is <br />generally less detrimental to plants than dewatering a losing stream (Kondolf et al. 1987). <br />One common long-term responses of vegetation to reduced water levels in a stream is for <br />the zones of vegetation parallel to the stream shift downward and towards the formerly active <br />chamlel (Ruble et al. 1994, Nilsson and Svedmark 2002). <br />Individual species' tolerance for low moisture can determine dominance of certain species under <br />conditions of low flow. Species with the capacity to withstand periods of drought or low flow <br />may become dominant during drought or if water limitation persists. An example in Western <br />riparian systems is the abundance of tamarisk relative to cottonwood in systems in which water <br />is limiting or flood magnitudes have been significantly modified. Not only is tamarisk more <br />drought tolerant than many other riparian species, but tamarisk is also tolerant of saline soils, <br />which can limit native riparian species such as cottonwood. High salt levels in the soil exacerbate <br />water stress of species not specifically adapted to salinity. Whereas tamarisk can gernunate and <br />grow in soils with extremely high salt levels, many native colonizers of riparian areas (i.e., <br />cottonwood and willow) are inhibited by high salt concentrations (Shafroth et al. 1995). <br />Seedlings of native species (cottonwood and willow) are most vulnerable to high concentrations <br />of salt, but adults may persist and spread by vegetative means (such as by rhizomes and root <br />sprouting). High salinity in valley bottoms and along streams is often associated with <br />evaporation from shallow water tables. As was previously mentioned, in systems that experience <br />occasional overbai~lc flooding, lower salt levels may be maintained due to periodic flushing of <br />floodplain substrate. <br />klcreasing the stage and frequency of low flows relative to natural conditions can lead to <br />encroachment and persistence of water-loving (hydrophytic species) near the channel or those <br />species that can spread by roots, stolons, rhizomes or nulners. Lush, marsh-like communities that <br />form in areas with persistent high water tables may provide productive, high quality habitat, <br />nutritious forage, and perform other beneficial functions (such as nutrient uptake; Stevens et al. <br />1995, Merritt and Cooper 2000). Elevated low flows can lead to higher rates of <br />evapotranspiration of riparian vegetation and more water loss directly to evaporation from the <br />46 <br />