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<br />. <br /> <br />. <br /> <br />" <br /> <br />Area Manager <br />DRAFT <br /> <br />29 <br /> <br />. Dams and Reservoirs. Most of the major and many of the minor Southwestern streams <br />that likely supported flycatcher habitat are now dammed (U .S. Fish and Wildlife Service <br />2002). Operation of dams modifies, reduces, destroys, or increases riparian habitats both <br />downstream and upstream of the dam site. Below dams, natural hydrological cycles are <br />modified. Maximum and minimum flow events can both be altered. Flood flows are <br />reduced in size and frequency below many dams. Base flows can be increased or <br />decreased depending on how the dam is operated. High flows are often reduced or <br />shifted from that of the natural hydrograph below dams managed for downstream water <br />supply. Daily water fluctuations can be very high below dams operated for hydroelectric <br />power. The more or less annual cycle of base flow punctuated by short duration floods is <br />lost. Thus, dams inhibit the natural cycles of flood-induced sediment deposition, <br />floodplain hydration and flushing, and timing of seed dispersal necessary for <br />establishment and maintenance of native riparian habitats. <br /> <br />Lack of flooding also allows a buildup of debris, resulting in less substrate available for <br />seed germination, and increasing the frequency of fires. Because ofreservoir <br />evaporation, natural levels of salt and other minerals are often artificially elevated in <br />downstream flow and in downstream alluvial soils. These changes in soil and water <br />chemistry can affect plant community composition. Upstream of dam sites, riparian <br />habitats are inundated by reservoirs, such as beneath Lake Powell, where Behle and <br />Higgins (1959) considered the flycatcher to be common. In some locales, this effect is <br />partially mitigated by temporary development of riparian habitats at inflow deltas, where <br />source streams enter the reservoirs. However, these situations tend to be vulnerable, <br />often inundated or desiccated as reservoir management raises and lowers <br />the water level, resulting in unstable flycatcher populations. Although large flycatcher <br />populations do occupy reservoir habitat, they may not be as numerous or as persistent as <br />those that occupied miles of pre-dammed rivers (U.S. Fish and Wildlife Service 2002). <br /> <br />. Diversions and Groundwater Pumping. Surface water diversions and groundwater <br />pumping for agricultural, industrial, and municipal uses are major factors in the <br />deterioration of flycatcher habitats (Briggs 1996). The principal effect of these activities <br />is simple reduction of water in riparian ecosystems and associated subsurface water <br />tables. <br /> <br />. Channelization and Bank Stabilization. Southwestern riparian ecosystems have also been <br />modified through physical manipulation of stream courses. Channelization, bank <br />stabilization, levees, and other forms of flow controls are carried out chiefly for flood <br />control. These engineering activities affect riparian systems by separating a stream from <br />its floodplain. These control structures prevent overbank flooding, reduce the extent of <br />alluvial-influenced floodplain, reduce water tables adjacent to streams, increase stream <br />velocity; increase the intensity of extreme floods, and generally reduce the volume and <br />width of wooded riparian habitats (Szaro 1989, Poffet al. 1997). <br /> <br />· PhreatophV!e Control. In some areas riparian vegetation is removed from streams, canals, <br />and irrigation ditches to increase watershed yield, remove impediments to streamflow, <br />