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34 important, particularly as it affects the interaction between introduced fishes and the endemic Colorado River fishes. Because these endemic fishes have evolved under natural conditions of high turbidity, it is concluded that the retention of these highly turbid conditions is an important factor for these endangered fishes. Reduction of turbidity may enable introduced species to gain a competitive edge which could further contribute to the decline of the endangered Colorado River fishes. Water clarity may affect the toxicity of PAHs. Certain PAHs (e.g., flouranthene) are photoactivated by ultraviolet light and become much more toxic than the nonactivated compound. Tests on larval/fry Colorado squawfish, razorback sucker, and other fish species at the National Biological Service's Midwest Science Center have indicated that in the presence of ultraviolet light, the toxicity of a particular concentration of PAH compound can increase by an order of magnitude or more (Mount 1995). Physical Habitat Seasonally flooded habitats such as vegetated shorelines, side channels, mouths of ephemeral washes, and tributaries have been identified as important during runoff as staging areas for Colorado squawfish and razorback sucker (Ryden and Pfeifer 1995a, 1995b, Miller 1994). Bliesner and lamarra (1994) found that overbank flooding occurs in the San Juan River at approximately 5,000 cfs and the area of inundation increases with higher flows. Full Project depletions would reduce volume and duration of peak flows during spring runoff in wet to average years and could reduce these seasonally flooded habitats. Studies are not yet complete, so specific impacts to these habitats cannot be identified. Reduction of runoff flows due to project operation also could impact the maintenance of spawning habitat. Current studies are ongoing to try to determine the relationship between annual hydrograph and the availability of spawning substrates. Biological Environment Data collected by Osmundson and Kaeding (1991) indicated that during low water years nonnative minnows capable of preying on or competing with larval endangered fishes greatly increased in numbers. No specific data is available relating nonnative fishes with flow regimes on the San Juan River, however, Reclamation concluded in the biological assessment that depletions by the Project that reduce peak spring flows may contribute to enhancement of nonnative species. In summary, the proposed project would reduce the amount of water delivered to critical habitat; alter the hydrologic regime; and increase water quality problems by reducing dilution. Without actions taken to offset impacts, further flow reductions in the San Juan River are likely to jeopardize the continued existence of the Colorado squawfish and razorback sucker and adversely modify or destroy their critical habitat. The San Juan River is an essential component of the Colorado River Basin and is needed to ensure maintenance of populations of Colorado squawfish and razorback sucker in the