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GREEN RIVER STUDY PLAN 32 February 2, 2007 5: Evaluate the effect of base flow variability on backwater habitat maintenance and quality. Description This study should begin as a synthesis of physical and biological information already collected in Reaches 2 and 3, including evaluating potential links between past and recent physical measurements and Colorado pikeminnow age-O monitoring. This ongoing work should be evaluated to refine, as necessary, studies to gain a better understanding of how base flows and base flow variability affect backwater maintenance and quality in Reaches 2 and 3. Western's annual studies of backwater topography in the Ouray reach are relevant to this evaluation and should be used to determine how base flow variability affects physical habitat characteristics (depth, volume, surface area). Integration of the backwater topography information with concurrent age-O Colorado pikeminnow monitoring should be explored as a way to link biological information with backwater variability. Upon completion.of existing data synthesis and integration, the need for continuation of studies or additional studies to quantify other habitat characteristics (e.g., temperature and productivity) should be determined. Past studies have documented fish communities in backwater habitats, but there has been little integration of these data, and little attempt to determine the relationship between fish communities and flow. This recommended study should synthesize physical and biological information already collected on backwaters to better understand physical habitat relationships and fish communities. Age-O monitoring currently collects samples of fish from backwaters. Project 144 (native response to nonnative control) supplements age-O Colorado pikeminnow monitoring with additional information on fish communities in backwaters. Following completion of analyses of data from these studies, a decision should be made regarding the need for additional or continuing studies to fill information needs and address uncertainties. Hypotheses to Be Evaluated and Information Needs to Be Filled · Effect of base flow variability (within-day, within-season, within-year, between years) on backwater quality in Reach 2 (U18). · The effect of base flows on nonnative fish populations in Reach 2 (U21). · Base flows in summer and autumn scaled to hydrologic condition favor formation of backwaters in Reach 2 (A8). · Maintenance of mean base flow within recommended levels of season and daily flow variability will promote favorable backwater conditions in Reach 2 (A9). · The effect of base flow variability (within-season, within-year, between years) on backwater habitat quality in Reach 3 (U24).