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c. Lake Powell: Lake Powell is a warm meromictic lake with a high degree of thermal and chemical stratification. It has a surface area that covers 255 square miles and contains up to 27 million acre feet of water. At full pool, depth of the reservoir near the dam is 561 feet. The thermocline changes seasonally, but below approximately 150 feet deep, the hypolimnion is consistently maintained due to thermal and chemical properties. Lake Powell exhibits a trophic gradient from the shallow productive inflows where nutrients and sediments are delivered by rivers,. to the nutrient poor clear water by the dam. This type of trophic gradient exists in each dendritic arm of the lake, as well as the main river channels. As the reservoir gradually shallows moving uptake from the dam, the depth and extent of the thermocline and hypolimnion change. Lake elevation can change drastically year to year depending upon snowmelt and runoff. Recent information suggests hydraulic processes can deliver the spring runoff plume at a mid- depth extending the entire length of the lake. The clear water reservoir offers habitat beneficial to the proliferation of nonnative fish. Generally, the reservoir is oligotrophic; deep, clear, and low in nutrient and chlorophyll abundance. d. Lees Ferry Tailwater: Water exits Lake Powell from the Glen Canyon Oam penstocks deep in the reservoir at elevation 3470 feet, creating a cold tailwater with a constant temperature near 46° F. The tailwater favors cold water species, like rainbow trout, which were not previously found in this reach of the Colorado River. The cold temperatures, coupled with the dam penstocks, limit fish movement downstream. Lake plankton is transported downstream to add to the food chain in the tailwater. e. Perennial/intermittent streams: Many side canyons contain springs and streams which may offer habitats for fish. Inventories of these habitats will be required before their true importance is known. B. BIOLOGICAL AND ECOLOGICAL RELATIONSHIPS 1. Pre-dam History of Ecosystem and Fishery: Specially adapted native fish prospered in the harsh Colorado River environment. Razorback sucker, Colorado squawfish, flannelmouth sucker, bluehead sucker, and two or three species of chub (Giles) were coe~only found in the river. Commercial fisheries were supported by "white salmon" (Colorado squawfish) and razorback sucker. Some nonnative fishes were present in the river flowing through Glen Canyon but, with the exception of carp and channel catfish, most were confined to backwaters, side channels and small perennial tributaries. Plains killifish, mosquitofish, fathead minnow, largemouth bass, bluegill, black bullhead, and green sunfish were reported in preimpoundment fish surveys. Before 1960 vehicle access to Glen Canyon was limited, except an unpaved road to Lees Ferry. Angling was confined to this access point, unless by the occasional rafters as they floated the rivers. 11