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29 <br />The creation of complex channel habitat and the formation and eventual <br />abandonment of channel features from which floodplain depressions are <br />formed occur primarily during spring runoff when flows are of <br />sufficient size and duration to cause major changes in channel <br />morphology through significant erosion and deposition of bed and bank <br />materials. The reduction in the magnitude, duration, and frequency of <br />high spring flows has slowed the rate at which channel morphology <br />changes. Consequently, the creation of complex channel habitat and <br />floodplain depressions has slowed. The placement of riprap and other <br />bank stabilization measures and the construction of dikes and levees <br />impede changes in channel morphology and contribute to the slowed <br />creation of complex channel habitat. In addition, the construction of <br />dikes and levees reduces existing channel habitat complexity by <br />causing channelization of the river. Dikes and levees also isolate <br />existing floodplain depressions from the channel during high flows. <br />The slowed creation of complex channel habitats and new floodplain <br />depressions, the reduction of existing channel habitat complexity, and <br />the isolation of existing floodplain depressions have acted to reduce <br />the quantity and quality of important habitat for endangered fishes. <br />Habitat N,aintenance <br />Backwaters, used by various life stages of endangered fish, are <br />damaged by the deposition of fine sediments which reduces their depth <br />and consequently their duration and frequency of inundation. Gravel <br />and cobble substrates, used by for spawning, are damaged by the <br />infiltration of fine sediments. The establishment of vegetation on <br />backwater sediments and on bars further reduces the value of these <br />habitats for endangered fishes. Furthermore, higher flows are <br />required to flush sediments from vegetated backwaters than from <br />unvegetated ones. Osmundson and Kaeding (1991) reported observations <br />that, on the 15-mile reach during the drought years of 1988 to 1990, <br />backwaters were filling in with silt and spring flows were not <br />sufficient to flush out the fine sediment. Also, they reported that <br />tamarisk colonized sand and cobble bars. The lower frequency of high <br />water years, therefore, decreases the frequency at which silt and sand <br />is flushed from backwaters, fine sediments are flushed from <br />gravel/cobble substrates, and vegetation is scoured from backwaters <br />and bars. As a result, the frequency at which these habitats are <br />suitable for use by endangered fishes has decreased. <br />Seasonal Habitat Availability <br />Summer (August-October): Osmundson et al. (1995) reported that <br />availability of habitats did not differ significantly between periods <br />of moderate flows and low flows. Though absolute area of habitat <br />decreases with declining flows, relative area or percent composition <br />of habitat types changes little. However, habitat use patterns did <br />change. The fish used a greater variety of habitats during moderate <br />flows than during low flows. During moderate flows, the fish used <br />J <br />