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C> '--:' (." 6 ., ~ collections in the San Juan River drainage, which indicated that Colorado squawfish once CO inhabited reaches above what is now the Navajo Dam and Reservoir near Rosa, New Mexico. Since closure of the dam in 1962 and the accompanying fish eradication program, physical changes (flow and temperature) associated with operation of the Navajo Project have eliminated Colorado squawfish in the upper San Juan River, both from the reservoir basin as well as from several miles of river downstream of the dam. The life-history phases that appear to be most critical for the Colorado squawfish include spawning, egg fertilization, and development of larvae through the first year of life. These phases of Colorado squawfish development are tied closely to specific habitat requirements. Natural spawning of Colorado squawfish is initiated on the descending limb of the annual hydrograph as water temperatures approach 200 Celsius (C). Spawning, both in the hatchery and in the field, generally occurs in a 2-month timeframe between July 1 and September 1, although high flow water years may suppress river temperatures and extend spawning in the natural system into September. Conversely, during low flow years when the water warms earlier, _spawning may occur in late June. A natural hydrograph with a large spring peak; a gradually declining/descending limb into early summer; and low, stable flows through summer, fall, and winter are thought to create the best habitat conditions for endangered fishes while maintaining the integrity of the channel geomorphology. Tyus and Karp (1989) pointed out the importance of peak flows (spring runoff) associated with reproductive activities of Colorado sQuawfish. They further stated that alteration of this hydrological event may affect initiation of Colorado squaw fish migration and spawning. Additionally, maintenance of low stable flows in summer and fall are necessary for growth and survival of young Colorado squawfish. Miller et al. (1982) concluded from collections of larvae and young-of-year below known spawning sites that there is a downstream drift of larval Colorado squawfish following hatching. Extensive studies in the Yampa and upper Green Rivers have demonstrated downstream distribution of young Colorado squawfish from known spawning areas (Archer et al. 1986: Haynes et al. 1985). Miller et al. (19821 also found that young-of-year Colorado squawfish; from late summer through fall, preferred natural backwater areas of zero velocity and less than 1.5-foot depth over a silt substrate. Juvenile Colorado squawfish habitat preferences are similar to that of young-of-ye'lr fish, but they appear to be mobile and more tolerant .of lotic conditions away from the sheltered backwater environment. Only two Colorado squawfish confirmed spawning siteS", as defined in the Colorado Squawfish Recovery Plan, have been located in the Upper Basin: river mile 16.5 of the Yampa River and river mile 156.6 of the Green River. These areas have the common characteristics of coarse cobble or boulder substrates forming rapids or riffles associated with deeper pools or eddies. It is believed that a stable, clean substrate is necessary for successful spawning and incubation. Substrates are swept clean of finer sediments by high flows scouring the bed prior to the spawning period (O'Brien 1984). As a result of the 1991 Biological Opinion issued by the Service for the proposed Animas-La Plata Project, and continued through reinitiation -in 1995 and a second Biological Opinion in 1996, Reclamation agreed to fund approximately 7 years of research on the San Juan River and its tributaries. While these studies are not yet complete. annual reports indicate that a