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13 Habitat Classification Habitats were classified into 6 geomorphological types, as defined in the appendix (Table A1). ~Vlost habitats were associated either with secondary channels (SC) or migratory sandwaves (MS). Other types of habitat such as those associated with horseshoe vortexes (HS) or flooded tributaries (FT) were relatively uncommon. Furthermore, the vast majority were. true backwaters rather than isolated pools or azeas with flow, so results were pooled for backwaters. Selectivity analysis of Colorado squawfish use of habitat by geomorphological type revealed a preference for Secondary Channel (SC) habitats. In the Green River NH data, chi-squared analysis showed a clear preference for SC habitats, which was significant for all seasons. Although the same analysis of Colorado River data also showed a preference for SC, it only approached significance for the lower;. section, in fall (Table 7). Colorado squawfish used all the habitats to some extent. In particular, HS habitats were used in a greater proportion than they occurred, particularly in the upper section where they were more frequently encountered. We observed in the field -that juvenile squawfish seemed to preferentially use HS habitats, but too few were collected to provide an analysis. Most other species, particularly the NNC, also used the SC habitats in at least the same proportion by which the habitats occurred. No preference was determined, because NNC were present in almost every habitat. However, densities (catch rates) of NNC in the SC habitats were usually higher than the MS habitats. In any case, the NNC overwhelm squawfish in pure numbers in all habitats. In general, SC habitats were lazger in azea and volume, and deeper on average and by maximum depth than MS habitats, and provided the majority of the available habitat (Table 8). Usually, the average maximum SC temperature was less than MS habitats in low/medium water years, and greater in high water years, but this was not a consistent phenomenon. The amount of SC habitat available was less closely correlated with peak flows and sampling flows than total habitat availability.