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<br /> <br /> <br /> <br /> <br />1 <br /> <br /> <br /> <br />1 <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />STUDY AREA <br />The Green River basin drains portions of southern Wyoming, eastern Utah, and <br />northwestern Colorado (Fig. 1) and the Green River is the largest tributary of the Colorado River. <br />Yampa Canyon in the lower Yampa River (river kilometer [RK] 0-74), Whirlpool and Split <br />Mountain canyons (RK 555-515) in the upper Green River, and Desolation and Gray canyons <br />(RK 340-211) in the middle Green River have high gradient and mixed cobble and sand <br />substrate. A valley reach in the middle Green River (RK 515-340) and Stillwater and Labyrinth <br />canyons in the lower Green River (RK 211-0) have lower gradient and substrate dominated by <br />sand and silt with small amounts of cobble. Discharge in the mainstem Green River upstream of <br />the Yampa River has been regulated since 1963 by Flaming Gorge Dam. During 1964-1996, <br />releases typically ranged from 22.6-130 m3/s, but were sometimes higher because of spillway <br />releases in high-water years (e.g. 1983, Tyus and Haines 1991). During the same period, <br />discharge in downstream reaches of the Green River in spring and early summer was high and <br />originated mostly in the Yampa River. In contrast, Green River discharge in late summer, fall, <br />and winter was lower and dominated by releases from Flaming Gorge dam. Discharge in the <br />highly variable and unregulated Yampa River was occasionally 566 m3/s in spring but sometimes <br />declined to < 2 m3 /s in late summer (U. S. Geological Survey records, gage 09251000). As a <br />result of an agreement between dam operators and the U. S. Fish and Wildlife Service, releases <br />from Flaming Gorge Dam since 1985 have maintained discharge of the Green River near Jensen <br />at a minimum of 51 m3/s +25% from July through September to maximize backwater habitat for <br />age-0 Colorado squawfish (Pucherelli et al. 1990). During 1979-1995, mean July-August <br />discharge of the middle Green River at Jensen, Utah (gauge # 09261000), and the lower Green <br />River at Green River, Utah (gauge # 09315000), was highly correlated (r = 0.99) and discharge <br />was higher downstream (mean = 141 m3/s, SD = 108 m3/s) than upstream (mean = 100 m3/s, SD <br />= 73 m3/s) mainly because of discharge from the White River. <br />4 <br />