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above river level. The upper Yampa Canyon, RM 45 (RK 72) to RM 20 (RK 32), is <br />characterized by several long sections interrupted by side canyon flood <br />deposits (containing boulders) that generally result in riffles or rapids. At <br />RM 20 (RR 32), the Yampa River enters the Weber sandstone and the <br />boulder/cobble substrate gradually picks up increasing amounts of sand. The <br />lower Yampa River is relatively deep and slow-moving with many meanders, <br />except for Warm Springs Rapid at RM 4.1 (RR 7). <br />Climatic conditions in the upper Colorado River Basin are highly variable <br />because of topography. Annual precipitation ranges from about 1.3 m (50 in) <br />in the mountains to less than 0.2 m (6 in) in and valleys. This results in <br />wet, cool, high mountain climate, widely fluctuating temperatures in low <br />mountains, and dry plateaus, and interior valleys which are hot in summer and <br />very cold in winter (Iorns et al. 1965). These conditions result in a mixture <br />of plant communities in the Green Subbasin ranging from alpine meadows and <br />aubalpine and montane forest to mountain brush, pinyon-juniper forest, <br />sagebrush, greasewood, and grassland communities. <br />The geological history of upper Colorado River Basin is recounted from <br />exposed deposits ranging from Precambrian to present. John Wesley Powell <br />(1961), who explored the Green River in 1896, described the geologic origin of <br />the river. The existing system has persisted relatively unchanged since the <br />Pliocene epoch and has been isolated from surrounding basins for millions of <br />years (Behnke and Benson 1980). This long period of development and isolation <br />has been reflected in the unique fish fauna. An appreciation of the long <br />period of adaptation by the native fish fauna is gained from the, statement by <br />Carlson and Carlson (1982), that Homo sapiens only dates back about 35,000 <br />9 <br />