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1 1 '~ 1 t 1 I habitats. Nonnative cyprinid densities did increase with higher amounts of total habitat in the summer, likely due to the higher temperatures, earlier onset of reproduction and longer growing season also found in low water years. Temperatures in accumulated degree days were found to influence Colorado pikeminnow growth and overwinter survival, with the highest values in years with the highest accumulation. Accumulated degree days from May through October were highest in the low flow years. In low flow years, relatively few larval pikeminnow are transported into nursery habitat areas (Bestgen et al. 1998), but those that are present experience better growth and overwinter survival than other years. Although maximum SC backwater temperatures were higher than the main channel maximum, average temperatures in SC habitats were lower than the average main channel temperature, due to the cooling effect at night. In order for Colorado pikeminnow to maximize exposure to the higher temperatures, they would have to migrate into backwaters during the day, and out at night. In the Mineral Bottom reach, the low water years of 1992 and 1994 were associated with high habitat volume and area, high temperatures, and high Colorado pikeminnow growth rates and overwinter survival. This might suggest that low water years would also result in high numbers of pikeminnow present in the fall and following spring, which does not occur, partly due to the relatively low transport abundance in low water years. However, another confounding factor is the presence of nonnative cyprinids. The increase in habitat availability, higher temperatures, earlier onset of reproduction and longer growing season in low water years contribute to the increased densities of these species. At Ouray, low water conditions were equally conducive to nonnative cyprinids, and catch rates were always higher than at Mineral Bottom. However, contrary to Mineral Bottom, high peak flows at Ouray created floodplain habitat, which led to even further increases of nonnative cyprinids. Although at Mineral Bottom high numbers of nonnative cyprinids in the summer had a generally negative effect on Colorado pikeminnow densities in the fall, there was no relationship seen at Ouray. It is possible that the negative effects of nonnative cyprinids can offset the positive effects of increased habitat and growth in low water years. Negative impacts of nonnative cyprinids probably include competition and predation (Muth and Snyder 1995; Ruppert et al. 1993; Beyers et al. 1994). In moderate flow years with peals spring flows near average, such as 1993 and 1996 in this study, both Colorado pikeminnow larval transport abundance (Bestgen et al. 1998), and densities of YOY pikeminnow in the summer and fall were maximized. Although habitat availability was not maximized and nonnative cyprinid densities were fairly high, 1993 and 1996 had the highest densities of YOY Colorado pikeminnow in the age one class the following spring. High water years such as 1995 can have low larval production, low habitat availability, and low temperatures resulting in low densities of Colorado pikeminnow in the fall and following spring. However, high water years can also negatively affect densities of nonnative cyprinids. This was true for peak flows that were over bar top, but less than bankfull in both the Mineral Bottom and Ouray reaches. This temporary suppression of nonnatives could create a window of opportunity the following summer for the newly hatched Colorado pikeminnow before the nonnatives begin reproducing again. The data collected and summarized in this study are highly variable, yet general results from the Ouray and Mineral Bottom components of the studies were quite similar. In both areas, total habitat availability at base flows were reduced in high flow years and increased in low flow xii