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by. Gray Canyon which contains larger and deeper pools than are found in Desolation Canyon. Other habitats within the canyon include eddies, rapids, and riffles. Side channels and backwaters also exist here. Substrate in Gray Canyon is composed mainly of boulder/rubble/sand with some gravel. In total, the river runs 159 miles through these two canyons. The Green River meets the Price River in lower Gray Canyon before that canyon ends at Book Cliffs escarpment (Tyus et al. 1987). The river then flows through the Green River Valley, near Green River, Utah. Predominant substrate in this valley varies from gravel/rubble to sand and silt. Few backwater habitats are found along this stretch of the Green, but riffles, side channels, and eddies do exist. At the southern end of the valley the San Rafael River joins the Green River before the river enters Labyrinth and Stillwater Canyons within Canyonlands National Park. This section of the river is dominated by a sand/silt substrate and is characterized by side channels and eddies. No rapids and few riffles exist within Stillwater and Labyrinth Canyons (Miller et al. 1982c). The river joins the upper Colorado River within Canyonlands National Park. Relationship to Constituent Elements: Water Because of water depletions which occur above this reach, historic water levels are seldom if ever obtained. Because of this, flooding of bottomlands is infrequent. Research to determine actual flow needs for this reach is currently underway as part of the Upper Basin Colorado River Implementation Program. Water depletions have the potential to reduce water volumes in this reach by about 30 to 40 percent (USFWS 1992b). Physical Habitat This reach does not contain the large number of backwaters that are present in the Green River from Sand Wash up to the Yampa River. However, large bottomlands which were historically flooded are present throughout this reach. Eddies and other low-velocity habitats also are present. Cobble/gravel bars which can provide spawning habitat occur within this reach; but no spawning locations have been confirmed. Biological Environment Very little is known on the quantity or quality of the food supply in this reach. Sources of input include the river above and from washes and side channels. The flooded bottomlands along this reach were probably once sources of food into the system but are seldom flooded the way the system is presently managed. Large concentrations of nonnative fishes in this reach include common carp, channel catfish, fathead minnow, and red shiner. 53