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SUMMER AND WINTER FLOW NEEDS Habitat Area Summer Habitat Use and Availability Squawfish use of habitats during summer differed between periods of moderate-flow and low-flow conditions (Fig. 13). When mesohabitat use data were partitioned into two categories, those locations made during summer flow levels of 1,378-2,368 cfs and those made during flows of 268- 931 cfs, different patterns of habitat use emerged. During times of moderate flow levels, the fish used a greater variety of habitats. Backwaters, eddies and pools collectively accounted for a mean use of 90%, while mean run use was only 10%. In contrast, during low water conditions, no fish were found in backwaters, eddies, or pools; mean use of slow and fast runs, on the other hand, was 97%. Relative availability of habitats did not differ significantly between periods of moderate-flow and low-flow conditions (Fig. 14). Percent total area for each habitat type was averaged across those flows that most closely corresponded to those that occurred at the time when fish-use data were collected. For the moderate-flow period, maps made at 1,240, 1,530, 1,630 and 2,870 cfs were used to calculate mean percent area; for the low-flow period, 557- and 810-cfs-maps were used. Though absolute area decreases with declining flows, relative area or percent composition evidently changes little. At both moderate and low summer flow levels, slow runs accounted for 54% of the total water area Riffles made up 22% of the total water area during moderate flows, and 25% at low flows. The greatest change in percent composition was for fast runs and pools. Fast runs made up 13% of the TWA at moderate flows and 9% at low flows. As flows dropped, pools increased from 4.5% to 7.5% of TWA. Thus, the change in habitat use between a moderate- and low-flow period without a corresponding change in relative habitat availability indicates that other factors also play a role in whether a particular habitat type will be selected. Though backwaters, eddies and pools are still present, and therefore available during low-flow conditions, their lack of use suggests that attributes of these habitats, either physical or biological, have changed and are no longer found desirable by the fish. Conversely, runs and slow runs are readily available at moderate flow levels and yet are little used then. Our habitat mapping only provides information on habitat quantity, not quality. Two physical attributes that would affect habitat quality were examined; these were habitat depth and habitat diversity (discussed later). Summer Habitat Preference (Objective No. 2) Backwaters, eddies and pools were preferred habitats during moderate summer flow levels in the 15-mile reach (Fig. 15). The mean preference rating for eddies was highest followed by pools and backwaters. Slow and fast were also used but were not preferred. No use of rapids was observed and gravel pits were unavailable during summer (Fig. 11). 28