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13 • <br />reduces existing channel habitat complexity b} causing channelization of the river. Dikes and <br />levees also isolate existing floodplain depressions from the channel during hieh flows. The <br />slowed creation of complex channel habitats and new floodplain depressions, the reduction of <br />existing channel habitat complexity, and the isolation of existing floodplain depressions have <br />acted to reduce the quantity and quality of important habitat for endangered fishes. <br />Habitat Maintenance <br />Backwaters, used by various life stages of endangered fish, aze damaged by the deposition of fine <br />sediments which reduces their depth and consequently their duration and frequency of <br />inundation. Gravel and cobble substrates, used by pikemirmow for spawning, are damaged by <br />the infiltration of fine sediments. The establishment of vegetation on backwater sediments and <br />on bars fiutlter reduces the value of these habitats for endangered fishes. Furthermore, higher <br />flows are required to flush sediments from vegetated backwaters than from unvegetated ones. <br />Osmundson and Kaeding (1991) reported observations that, on the 15-mile reach during the <br />drought years of 1988 to 1990, backwaters were filling in with silt and spring flows were not <br />sufficient to flush out the fine sediment. Also, they reported that tamarisk colonized sarid and <br />cobble bars. The lower frequency of high water years, therefore, decreases the frequency at <br />which silt and sand is flushed from backwaters, fine sediments are flushed from graveVcobble <br />substrates, and vegetation is scoured from backwaters and bazs. As a result, the frequency at • <br />which these habitats are suitable for use by endangered fishes has decreased. <br />Seasonal Habitat Availability <br />Summer (August-October): Osmundson et al. (1995) reported that, in the 15-mile reach, <br />availability of habitats did not differ significantly between periods of moderate flows and low <br />flows. Though absolute area of habitat decreases with declining flows, relative area or percent <br />composition of habitat types changes little. However, pikemirtnow habitat use patterns did <br />change. The fish used a greater variety of habitats during moderate flows than during low flows. <br />During moderate flows, the fish used primarily backwaters, eddies, and pools. During low flows, <br />the fish used slow and fast runs almost exclusively. The change in habitat use without a <br />corresponding change in relative habitat availability indicates that other factors also influence <br />habitat selection. These factors could include changes in quality of physical habitat features such <br />as diversity, depth, dissolved oxygen, etc., or changes in biotic interactions. Osmundson et aI. <br />(1995) interpreted the pikeminnow behavioral changes as reflective of suboptimal conditions; the <br />behavioral changes demonstrate the ability of the species to modify their habitat use patterns to <br />temporarily cope with adverse conditions and do not demonstrate habitat preferences under <br />optimum conditions. <br />Winter (November-March): Osmundson et al. (1995) reported that, in the 15-mile reach, flows <br />during the winter are usually moderate because no water is diverted for irrigation and because <br />additional water is released through upstream dams to increase reservoir storage capacity in . <br />anticipation of spring runoff. The relative availability of slow tuns and riffles during the winter <br />