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River enters the Green in lower Gray Canyon. The Green River then enters the Green River Valley near the town of Green River, Utah, and receives the San Rafael River about 40.2 km (25 mi) further downstream. The Green River then proceeds south through Labyrinth and Stillwater canyons (composed mainly of the Mesa Verde group) in Canyonlands National Park to loin the upper mainstem Colorado River. During its course, the Colorado River drops from about 4,300 m (14,000 ft) near its source to about 1,219 m (4,000 ft) at its mouth, a 3,048 m (10,000 ft) drop in elevation. Yampa Canyon, located in the last 45 mi of the Yampa River in DNM, hoe been studied since 1981 when a Colorado squawfish spawning site was discovered there (Tyus and McAda 1984). The Yampa River, with an average annual discharge of 1.5 million acre feet, arises on the western slope of the Rocky Mountains near Yampa, Colorado, and flows northward and then westerly for approximately 321 km (200 mi), before joining the Green River in DNM. Snow melt causes flows to rise in mid-to-late April. Daily averages range from 100-500 cfs during August-March, to an excess of 10,000 cfs in June. In the wincer of 1980-1981, precipitation and snow pack were much less than normal and 1981 spring runoff in the Yampa River was consequently reduced in bath duration and magnitude. In contrast, the winters of 1982-1983, and 1983-1984 were very wet and the 1983 and 1984 runoffs were characterized by large volumes and high peak flow, over 20,000 cfs each year. The study area also included the lower 161 km (100 mi) of [he Yampa River in 1963 for radiotracking Colorado squawfish. Zn 1984 and 1985, emphasis was placed on the lower BO km (50 mi) of the Yampa River, which lie within DNM. The Yampa River enters DNM at RM 48 (RK 77), and at RM 45 (RK 72) it enters a steep-walled canyon with near-vertical walls which rise 417.6 m (1370 ft) 8