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Ho 4: Overwinter survival of age-O Colorado pikeminnow is significantly higher in deep backwaters than in shallow backwaters. Deep backwaters may be more resistant to flow fluctuations that otherwise flood or desiccate shallow backwaters and render them unsuitable as nurseries. Antecedent flows that shape backwaters may be important if this relationship is demonstrated. "05: Overwinter survival of age-O Colorado pikeminnow is significantly higher in years of low densities of non-native fishes in nursery backwaters. Predation on age-O Colorado pikeminnow by non-native red shiners, fathead minnows, green sunfish, channel catfish, and black bullheads in nursery backwaters is documented or suspected (Ruppert et a1. 1993). Native/non-native fish interactions during winter have not been described, but it is believed that predator forces continue throughout winter to reduce abundance of age-O Colorado pikeminnow. STUDY AREA This study was conducted in two reaches of the Green River: (1) the lower reach from the Colorado River confluence (River Kilometer [RK] 0) upstream to Green River, Utah (RK 193), referred to as Reach 3 of the ISMP, and (2) the upper reach from Sand Wash (RK 346) upstream to Split Mountain (RK 515), referred to as Reach 4 of the ISMP (Figure 1). METHODS Hydrology and Temperature River hydrology and temperature data were procured for winters ofl987-88 to 1994-95 from the U.S. Geological Survey (USGS) stream gages at Green River, Utah (#09315000) and for winters of 1989-90 to 1994-95 from the gage at Jensen, Utah (#09261000). River ice conditions and hack water habitat dynamics were evaluated afield during the winters of 1993-94 and 1994-95 (Valdez and Cowdell 1999). Frequency of ice formation was determined from USGS field notebooks for 1946-92 (data provided courtesy of J.C. Schmidt, Utah State University). Age-O Survival Age-O Colorado pikeminnow were sampled in backwaters in fall and the following spring to assess changes in density for each year class, starting with fall 1987 and ending with spring 1995: F all sampling was conducted in late September and early October when the age-O fish were 3-4 months old and large enough to confidently identify afield and to be handled with a minimum of stress and mortality. Spring sampling was in late March and early April, when water temperatures were warming and prior to onset of spring runoff, which flooded nursery backwaters. Overwinter survival index was defined as the change in density of age-O Colorado pikeminnow in nursery backwaters over a 6-month period, from late September to late March. Data for 1987 and 1988 year 3