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<br />1 <br />1 <br /> <br /> <br /> <br />i <br /> <br />1 <br /> <br /> <br />1 <br /> <br />1 <br />1 <br /> <br /> <br /> <br />The relatively acid CRB produces a small amount of water for its size; approximately <br />30,000 m3/km2 of land area. (Bishop and Porcella 1980). In contrast, the comparably- <br />sized Columbia River Basin (670,000 km2) produces over an order of magnitude more <br />water; approximately 3$6,000 m3/km2 (United States Army Corps of Engineers 1999). <br />The annual hydrograph of the Colorado River and higher-elevation tributaries is <br />dominated by snowmelt-runoff occurring in spring; headwater azeas may receive up to <br />130 cm of precipitation that falls mostly as snow, whereas low desert areas typically <br />receive minimal (~30 cmlyr) precipitation (Bishop and Porcella 1980). Law-flow <br />periods generally occur in late summer and autumn through winter, after snowmelt- <br />runoffhas subsided. Occasionally, intense rainstorms can produce short-duration floods <br />of greater magnitude than snowmelt runoff peaks (Carothers and Brown 1991). In <br />addition to extreme seasonal flow fluctuations, the Colorado River system may also <br />exhibit longer-term variation in flow (wet and dry cycles) that span years or decades <br />(Bishop and Porcella 1980). <br />Other characteristics of the historical Colorado River system include high salinity and <br />sediment levels, especially in the lower basin, and water temperatures that seasonally <br />range from 0 to 30 °C (Carlson and Carlson 1982, Carlson and Muth 1989, Carothers and <br />Brown 1991). The historical combination of frequent drought and flood, wide ranging <br />water temperatures, and high sediment and solute loads created a harsh environment that <br />provided a unique setting for evolution of a distinct group of endemic fishes (Ono et al. <br />1983}. <br />Fish Fauna <br />The native fish fauna of the CRB includes 22 genera and at least 35 species, with <br />approximately 27 and 74°fo genus and species level endemism, respectively (Evermann <br />and Rutter 1895, Miller 1959, Carlson and Muth 1989, Valdez and Carothers 1998). <br />High levels of endemism in CRB fishes likely resulted from millions of years of geologic <br />isolation from surrounding watersheds and rigors of the physical environment (Behnke <br />Final Report September 2002 <br />4 <br /> <br />