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00097~ Status and Distribution Colorado pikemi~ow were historically distributed throughout warmwater r~aches of the Colorado River Basin .from Wyoming and Colorado south to the Gulf of California. By the 1970s they were extirpated from the entire lower basin (downstream of Glen Canyon Dam) and from portions ofthe upper basin as a result of major alterations to the riverine environment. Having lost some 75-80 percent of its former range, the Colorado pikemiim'Ow was federally listed as an endangered species in 1967 (Miller 1961, Moyle 1976, Tyus 1991, Osmundson and Burnham 1998). Colorado pikeminnow are presently restricted to the Upper Colorado River Basin and inhabit warmwater reaches ofthe Colorado, Green, and San Juan Rivers and associated tributaries. The species inhabits about 350 miles ofthe mainstei:n Green River from its confluence with the Colorado River upstr~am to the ~outh of the Yampa River. In the Yampa River, its rap.ge extends upstream an additional 160 miles. Colorado pikeminnow also occur in the lowermost 104 miles of the White ~ver, another tributary to "the Green River. In the mainstem Colorado River, distributit>uofthe species extends 201 miles upstream from the upper end of Lake Powell to Palisade, Colorado (Tyus 1982). Major declines in Colorado pikeminnow populations occurred during the dam-building era of the , 1930s through the 1960s. Behnke and Benson (1983) summarized the decline of the natural ecosystem, pointing out that dams, impoundments, and water use practices drastically modified the river's natural hydrology and channel characteristics throughout the Colorado River Basin. Dams on the mainstem broke the natural continuum of the river ecosystem into a series of disjunct segments, blocking native fish migrations, reducing temperatures downstream of dams, creating lacustrine habitat, and providing conditions that allowed competitive and predatory nonnative fishes to thrive both within the impounded reservoirs and in the modified river segments that connect them. The highly modified flow regime in the lower basin coupled with the introduction of nonnative fishes decimated populations of native fish. Major declines of native fishes first occurred in the lower basin where large dams were constructed from the 1930s through the 1960s. In the upper basin, the following major dams were not constructed until the 1960s: Glen Canyon Dam 'on the mainstem Colorado River, Flaming Gorge Dam on the Green River, Navajo Dam on the San Juan River, and the Aspinall . Unit Dams on the Gunnison River. To date, some native fish populations irithe Upper Basin have managed to persist, while others have become nearly extirpated. River segments where native fish have declined more slowly than in other areas ire those where the hydrologic regime most closely resembles the natural condition, where adequate habitat for all life phases still exists, and where migration corridors are 'unblocked and allow connectivity among life phases. In the mainstem Colorado River, the magnitude of spring flows has declined by 30-45 percent since the early part of the century (Osmundson and Kaeding 1991,. Van Steeter 1996, Pitlick et al. 1999). Such flow reduction negatively affects Colorado pikeminnow in four ways: 6