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RESULTS Hydrology and Temperature Reach 3 F all sampling of age-O Colorado pikeminnow in backwaters of Reach 3 for the period 1987- 95 took place in late September at main channel temperatures ofl6-250C, backwater temperatures of9-320C, and flows of 1,200-3,200 cfs (Table 1). Sampling in spring took place in late March at main channel temperatures of5-14 OC, backwater temperatures of5-220C, and flows of2,200-7,400 cfs. Hydrographs and thermographs (USGS gage at Green River, Utah) for the sample period show that flow in Reach 3 varied considerably from one year to the next, with winter flows typically between 1,000 cfs and 3,000 cfs, except for 1987-88 and 1993-94 when flows were higher (Figure 2). The 6- month period between samples (i.e., late September-late March) was typically the period of lowest flow, except for a rising hydrograph and spike flows which usually occurred in early March, as a result of early, low-elevation runoff. Water temperature in Reach 3 stayed near freezing ti-om mid-December to mid-February in nearly all years observed, except for winters of 1989-90 and 1993-94, when temperatures were often above freezing. Reach 4 In Reach 4, fall sampling took place in late September and early October 1989-93 at main channel temperatures oflO-220C. backwater temperatures of8-270C, and flows ofl,100-1,800 cfs (T able 1). Sampling in spring took place in late March and early April at main channel temperatures of 5-160C, backwater temperatures of 4-220C, and flows of 1,650-3,600 cfs. Hydrographs and th~l1nographs (USGS gage at Jensen, Utah) for the sample period show that flow in Reach 4 also \aried considerably from one year to the next, with winter flows typically between 1,000 cfs and 2.000 cfs, except for 1991-92 and 1993-94 when flows were higher (Figure 3). A rising hydrograph and spike flows in early March were not as dramatic in Reach 4 as in Reach 3. Water temperature in Reach 4 stayed near freezing from early December to early February in nearly all years observed, although in 1993-94, river temperatures remained above freezing, and in 1994-95, temperatures were variable, at and above freezing. Age-O Survival Reach 3 Overwinter survival index of age-O Colorado pikeminnow in Reach 3 ranged from 23% to 100% for the nine cohorts assessed (i.e., 1987-95), based on changes in geometric mean CPUE (Table 2). Decreases in CPUE were significant (Students '1', P>0.05) for seven of the nine cohorts. Decrease in CPUE for the 1990 cohort was not significant and fall density for the 1992 cohort was low, with a small increase in spring density, probably as a result of sample variation. Mean total lengths of age-O fish in fall and spring were similar and consistent for the first s~ven cohorts assessed (i.e., 1987-93) with ranges of 34-40 mm and 40-49 mm TL, respectively 6