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Spawning Temperatures and Discharges Flow conditions are extremely variable in the upper Colorado River " system throughout the year but at no time are they more dynamic than during the advent and conclusion of Colorado squawfish spawning. Observations show that spawning occurs during the period when spring runoff flows are rapidly declining, the timing and magnitude of which are determined by winter's snowfall and climatic conditions during the spring. There are two major parameters, discharge and temperature, which reflect fluvial conditions for which extensive data bases have been accumulated. How critical these parameters are to successful s pawning is ye t unclear but some general trends are emerging. A broad range of conditions have .been observed since 1979 with heavy snowfall and accompanying heavy spring runoff occurring in 1983 and 1984 and a very low flow year in 1981. Discharge and seasonal conditions are major influencing factors for temperatures, therefore considerable variation has been observed, though, again wi thin constraints. All the streams except the Yampa in which spawning is believed to occur have been significantly developed which has resulted in some departure from natural hydrologic conditions. This is particularly true for the upper Green River where the Flaming Gorge Reservoir has a major in- fluence on flow and water temperatures downstream. Colorado squawfish spawning appears to be synchronized with termination of the spring runoff period when flows are declining and temperatures are approaching the seasonal high. It can be reasoned that this behavior has evolved to assure optimum. conditions for young survival and growth. Being so variable a system, it has been difficult to develop clear correlations. for flow and temperature needs. In five years of study there have been no two years of similar conditons. Baynes et ale (1984) have observed the highest production of young Colorado squawfish during years of high spring flows with a substantial reduction in the production of non- native fishes. This uy be exceedingly important to native fishes in a altered ecosystem like the Upper Colorado River Basin. Haynes et a1. (1984) collected larval fishes at nearly an 800% higher rate in the Yaapa River during years when spring runoff was in excess 20,000 cfs. These, of course, are one in Za-year occurrences or more. 1 Q