GENERAL31841 - Laserfiche WebLink

Home Browse Search

7 the Colorado River upstream to the mouth of the Yampa River. In the Yampa River, its range extends upstream an additional 160 miles. Colorado pikeminnow also occur in the lowermost 104 miles of the White River, another tributary to the Green River. Tn the mainstem Colorado River, distribution of the 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 1930's through the 1960'x. 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 1930's through the 1960'x. In the upper basin, the following major dams were not constructed until the 1960'x: 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 in the 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 are 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 (Dsmundson and Kaeding 1991, Van Steeter 1996, Pitlick et al. 1999). Such flow reduction might negatively affect Colorado pikeminnow in. four ways: (1) reducing the river's ability to build and clean cobble bars for spawning; (2) reducing the dilution effect for waterborne contaminants from urban and agricultural sources that may interfere with reproductive success; (3) reducing the connectivity of main- channel and bottomland habitats needed for habitat diversity and productivity; (4) providing a more benign environment for nonnative fish and invasive, nonnative, bank-stabilizing shrubs (salt cedar) to persist and flourish (Osmundson and Burnham 1998). In general, the existing habitat has been modified to the extent that it impairs