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4 <br />have been reported from the Yampa River near Craig, Colorado (Tyus and Karp <br />1989) in a "Transition Zone". <br />lorns et al. (1965) divided the Upper Basin into the Grand, Green and San <br />Juan sub-basins. This naming system has since been adopted by numerous <br />authors and agencies. The Grand sub-basin refers to that area draining into the <br />mainstem Colorado River above the confluence with the Green River. The <br />Colorado River originates in Rocky Mountain National Park, Colorado near <br />LaPoudre Pass at an elevation above 10,000 feet. Major tributaries include the <br />Dolores, Gunnison, Roaring Fork, Eagle and Blue Rivers. The Grand sub-basin <br />makes up 23% of the area of the Upper Basin and contributes 44% of the virgin <br />Upper Basin streamflow (lorns et al. 1965). Prior to 1921 the portion of the <br />Colorado River above the Green River confluence was called the Grand River, thus is <br />the designation of this sub-basin as the Grand (Skogerboe 1982). <br />The Green River Basin comprises the Green sub-basin. The Green River <br />originates above timberline at the northern end of the Wind River Mountains in west <br />central Wyoming and flows through the Green River Basin to Flaming Gorge <br />Reservoir. Major tributaries include the San Rafael, Price, White, and Duchesne <br />Rivers in Utah, the Yampa River in Colorado, and the Blacks Fork in Wyoming <br />The Green sub-basin comprises 43% of the Upper Basin area, but contributes only <br />37% of the virgin Upper Basin streamflow (lorns et al. 1965). <br />The San Juan sub-basin consists of the San Juan River Basin and tributaries <br />discharging into Lake Powell or the Colorado River below the Green River-Colorado <br />1