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partitioning was made that was a composite of the ones made for each species. Transitional <br />periods were either lumped or split but the core months of each season stayed basically the <br />same. <br />A distinct winter period emerged in which averaged pool use for both species was <br />greater than 40% for all months and use of slow runs was 20-40%. Winter included <br />November, December, January, February and March. The core spring season included <br />April, May and June when use of pools averaged 30-40% and backwater use was 20-40%. <br />Summer included August, September and October. In October, the diversity of habitat use <br />declines and pool use increases for both species. However, slow run use is still high and <br />pool use is not nearly as high as during winter. Also, main channel temperatures are still <br />high enough in October for the fish to still be quite active. July appears to be more of a <br />transition month. Although habitat use for Colorado pikeminnow is fairly constant during <br />July, August and September, habitat use by razorback sucker in July is more similar to that <br />during May and June, particularly the continued high use of backwaters (36%). July was <br />included as one of the spring months not only because of the habitat-use pattern of <br />razorback suckers, but also because flow levels are still quite high from snowmelt runoff <br />during this time, having not yet returned to base flow levels. This results in a spring period <br />which includes the runoff months (April-July) and two base-flow periods, summer and <br />winter. <br />FLOW EFFECTS <br />Habitat Heterogeneity and the Creation and Maintenance of Mesohabitats <br />Fish habitat in rivers is largely controlled by the interacting factors of channel width, <br />depth, slope, substrate size and the surrounding topography (Lamarra 1999). Channel- <br />forming flows are high flows capable of eroding banks, moving large substrate particles, <br />shifting cobble and gravel bars, and scouring vegetation (Pitlick et al. 1999). When such <br />flows occur in unconfined reaches, where the river is free to move laterally, side channels <br />21