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. be used to compare the present condition of the Colorado River. Man's im- Table 6. Estimate of Habitat Change Due to Major Reservoir Construction in Principal Reaches of the Colorado River Basin pact on the environmental characteristics of the Colorado River Basin can be categorized into three broadly defined areas: (1) effects caused by water resource management diversions and consumptive uses (dams. hydro- Mil es electric power. flow control. inter- and intra-basin diversions); (2) ef- Reservoir length fects of various economic activities and land uses; and (3) effects of River length introducing alien floral and faunal species and of other changes in the 1~440 Colorado-Mainstream (Mexican border to headwaters) Laguna Dam Imperial Dam Lake Havasu Lake Mo jave Lake Mead Lake Powell 5 30 80 65 105 175 biotic community. Water Management. Because of reservoir development in the basin, particularly Flaming Gorge and Glen Canyon Dam (Lake Powell). the varia- ble flows and velocities of the river have been sharply altered through Green River Flaming Gorge Fontenelle 50 15 inundation of river reaches and flow alteration by reservoir releases (see table 6). The consequences of constructing the present dams have Gunnison Blue Mesa Reservoir and Morrow Point Reservoir 40 been to reduce the silt-carrying capacity of the rivers. reduce flows downstream. increase salt concentrations (from increased evaporation), San Juan Navajo Reservoir 35 change the types and quantities of biota residing in the river. and change Gila recreational potential (amount of use as well as type of use). Painted Rock San carlos Roosevelt Horse Shoe Barlett Lake Pleasant 25 25 30 10 15 10 715 Stream reaches below dams are affected by a variety of phenomena which result from the way water is released in the operation of the reser- voir. The releases from upper basin reservoirs tend to be extremely cold Total because the water comes from the lower strata of the reservoir. For ex- 500 130 .. ~ .. .. ~ ~ I 370 ~ 0( e I 805 .. - ... ... ... .. > ... ,.. 0( 3.245 ample. below Flaming Gorge Reservoir, these releases have resulted in a Note: 700 320b= 0.22; 22% of major reaches of river system dammed. shift from native fishes to a trout fishery because of the reduced silt load and because the water is too cold to support other types of fish. .