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r .. Table 1. Changes in the Upper Colorado River that Have or May Affect the River and the Fish. PHYSICAL Dams - Have changed riverine environment into reservoirs; Are obstructions to fish passage; Have changed natural streamflows and water temperatures. Streamflows - Regulated by releases from reservoirs; Reduced by diversions from structures for irrigation; Flushing flows may not be adequate for cleaning gravel or rubble substrates that are used for spawning; Affects the amount of fish habitat available as riffles, runs, pools, or backwaters. Water Diversions - Reduces the streamflows downstream that may become critical for rare fish during low runoff periods; May cause losses of larval, young-of-the-year and juveniles of rare fish; Return flows increase salinity and may carry toxic substances such as pesticides or herbicides. Water Temperature - Cold water releases below high dams reduce the water temperature which is suitable for cold-water species but not for the warm-water endemic species; Adversely affects the temperature required for spawning or growth of rare fish. CHEMICAL Water Quality - Salinity increases downstream through concentration of minerals in irrigation return flows (may not be very important in the upper basin at this time but is important in the lower basin); Contaminants (e.g., pesticides or herbicides used in agriculture or leaching of toxic substances such as heavy metals) may increase the mortality of endemic fish. BIULUGICAL Species Composition - Environmental changes have favored some species of invertebrates and fish while decreasing the abundance of others; Non-native fish have been introduced by intent or accident that may either compete with, or prey upon, endemic rare fish. Recreational Activities - Fishing pressure has increased that may cause additional mortality of rare fish that are very susceptible to angling; However, razorback suckers are not caught very readily by bait or lures that are presently used by anglers. 18