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physical pazameters is very difficult. However, even small increases in numbers of YOY Colorado squawfish in the upper river should be obvious. Tyus and Haines (1991) found no relationship between spring flows and relative abundance of YOY Colorado squawfish in the Green River; however, they did find significant correlations with summer flows. No significant correlations were found between ISMP data and various flow variables in the Green River. It may be that there is a threshold of spring flow (both peak and duration) that is reached in the Green River during most yeazs. This threshold may not have been reached in 1989-the yeaz of lowest runoff -and lowest CPE of YOY Colorado squawfish in the upper Green River. Growth Mean total length of YOY Colorado squawfish was between 35 and 45 mm in most reaches and most yeazs. More than 50% of the fish collected were between about 20 and 50 mm long in most yeazs. Tyus and Haines (1991) reported mean lengths of 29 to 47 mm for YOY Colorado squawfish collected from the Green River between 1979 and 1986. Vanicek and Kramer (1969) estimated an average length of 44 mm at the end of the first growing season using a combination of back calculation and empirical data. It was difficult to determine the maximum size of fish after the first growing season, because Colorado squawfish as long as 100 mm were collected in some years. It is likely that some of the larger fish collected in the fall aze actually completing their second growing season. However, because the length-frequency distribution does not exhibit a second peak, there is no way to know for sure without aging each fish. Valdez and Cowdell (1993) collected Colorado squawfish less than 40 mm long in spring samples. These smaller fish could still be less than 100 mm the following fall. Hawkins (1992) used back calculation of length from scale analysis to determine the average length of fish at the end of their second growing season to be 90 mm. Vanicek and Kramer (1969) also reported most fish to be between 70 and 100 mm long after their second growing season. As observed earlier, mean total length was smallest in 1986-the year of highest runoff observed in the monitoring program to date-and was positively correlated with accumulated water temperature in three of the four reaches sampled. This correlation between total length and accumulated temperature units is primarily a result of timing of the spawning period. In lower runoff yeazs, the water wazmed eazlier and therefore spawning occurred earlier. The young Colorado squawfish then had a longer growing season before sampling occurred than they did in years when spring runoff was higher and more extended. Although temperature is an important component of spawning period, other factors such as photo period may also play a role. Habitat Use During years of high abundance, YOY Colorado squawfish were found in almost all backwaters sampled with ISMP except for those in Reach 2. Variation in the percent of backwaters occupied during other yeazs was roughly proportional to their relative abundance in the river as a whole. Based on their presence or absence in backwaters and the limited physical measurements made at each backwater, there was no clear preference for one type of backwater over another by YOY Colorado squawfish. There were some significant differences among the data pairs comparing vazious habitat parameters, but the data were highly variable and most comparisons did not produce significant differences. Further, the significant relationships were not always in the same categories, although a trend was often detectable among the many observations. None-the-less, the significant differences that were observed give some general indications about habitat use by YOY Colorado squawfish: 21