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ECOLOGY AND CONSERVATION OF NATIVE FISHES IN THE UPPER COLORADO RIVER BASIN 173 genetic purity ratings ofB, B+, A- or A (i.e., slightly hybridized to essentially pure) are defined as con- servation populations for which management ac- tions are implemented, and in 2003 included to- tals ofl,648 km (1,024 stream miles) and 455 ha (1,124 acres oflakes). These results show that both pure and essentially pure populations are still present in many waters in Utah, Colorado, and Wyoming. Spawning occurs in spring in streams and tribu- tary inflows, and eggs hatch in 28-40 d at 8-12 oc. Colorado River cutthroat trout feed largely on plank- ton throughout life, and their growth is slower than that of Yellowstone cutthroat trout 0. clarkii bouvieri. Maximum growth in Trapper's Lake was in midsummer (Drummond 1966). Colorado River cutthroat trout is susceptible to common salmonid diseases, especially whirling disease Myxobolus cerebralis (Nehring 1998). Transmission of diseases to wild populations by hatchery stocks is recognized as the most significant threat, and policies and regu- lations in Wyoming and Utah address fish health status, disease certification of stocked and imported fish, and stocking protocols. Fish testing positive for whirling disease in Utah and Wyoming hatcheries are not stocked, and in Colorado, a policy clearly designates native cutthroat trout waters and other wild trout habitats that are negative for whirling disease as the most protected category. Threats to Native Fishes The following threat descriptions apply primarily to endangered and other native fishes in the main rivers and tributaries of the upper basin. Threats to the Kendall Warm Springs dace and Colorado River cutthroat trout are described under Current Status and Ecology and Species Comervation Programs. Flow Regulation and Diversion Flow regulation in the upper basin began in the mid- 1800s as small tributary impoundments and irriga- tion diversions. Ratification of the Colorado River Compact of 1922 by the seven basin states divided the Colorado River into upper and lower basins and allocated 7.5 million acre-feet ofwater to each basin, based on estimated annual flow of 16.8 million acre- feet for the period 1896-1921 (Fradkin 1981; Reisner 1986). However, average annual flow of the Colorado River, 1922-1976, was only 13.9 million acre-feet, and compact allocation was based on an overestimate of available water. The Upper Basin Compact of 1948 further apportioned water to each of the upper basin states by percentage of available annual volume (i.e., Colorado, 51.75%; Utah, 23%; Wyoming, 14%; New Mexico, 11.25%). All seven basin states faced the problem of constructing costly storage and delivery systems to realize their full com- pact allocation. Completion of Boulder (Hoover) Dam in 1935 marked the beginning of dam construction on the Colorado River. Thirteen main-stem dams regulate flow of the Colorado River and hundreds of smaller dams control virtually every stream in the basin. Major dams in the upper basin include Flaming Gorge on the Green River; Blue Mesa, Morrow Point, and Crystal (Aspinall Unit) on the Gunnison River; Dillon, Green Mountain, Shadow Mountain, and Ruedi on tributaries of the upper Colorado River; McPhee on the Dolores River; Taylor Draw on the White River; and Glen Can- yon on the main-stem upper Colorado River. The larger dams were completed in the early 1960s under authority of the Colorado River Storage Project Act of 1956. Glen Canyon Dam is the larg- est in the upper basin and is located 25 km up- stream of Lees Ferry, the Compact dividing point between upper and lower basins. Annual discharge from the upper basin at Lees Ferry exceeded 18 million acre-feet in 1929, and lowest discharge was only 4.4 million acre-feet in 1934. The historic upper basin was characterized by dramatic ann ual and seasonal flow variation (i.e., ex- ceptionally high flows in spring and early summer, and lower flows in late summer through winter). Flows typically began rising in March with low elevation snowmelt, were highest in late May and early June with snowmelt runoff, receded in late June and July, and were relatively low and stable from August through March, except for flow spikes from periodic storms. Year-to-year flow variation depends on moun- tain snowpack. Historic average annual flow at Lees Ferry, 1922-1962, was highly variable from about