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2 1 I , I 'J I 1 ~ 1 I 1 t I .'. I I ,I I I suitability for each cel1 in the cross-sectional grid. Habitat availability is measured by an index cal1ed weighted useable area (WUA), the summation of cell areas weighted by its suitability index. When plolled versus discharge WUA typically peaks at a single flow that is considered the flow that maximizes habitat. PHABSIM has been criticized because of assumptions implicit with micro-habitat suitability curves, assumptions of positive relationships between habitat availability and fish abundance, and the tendency of users to recommend flows at the peak of the WUA-discharge relation regardless of natural flow occurrence. These assumptions have been obstacles for using PHABISM to model impacts ofreduced flows on large warm water rivers of the west slope (Rose and Hahn 1989). The Meso-Habitat method of this study is also an integration of hydraulic and biologic analyses. We used a 2-D flow model to simulate depths and velocity. Fish habitat suitability was derived from density estimates made at each site. Two- dimensional (2-D) !low models have been found to have high potential for application in instream flow studies (Leclerc et al., 1995; Bovee, 1996; Kondolf et al. 2000). Two- dimensional models offer a significant improvement over one-dimensional (I-D) modeling by increasing spatial resolution. This allows for a highly accurate quantifkation of the physical habitat for each flow of interest. 2-D models are not dependent on micro-habitat suitability curves for predicting habitat availability. Meso- habitat scale precision is more efficient for sampling biological data. Criteria can be established for individual species and also by guilds (syrnpatric species) (Parasiewicz 200]). A relationship between habitat availability to fish abundance assumes that fish distribute themselves in the river primarily as a function of habitat. A large part of this study was to develop meso-habitat criteria for two native suckers. This was accomplished by examining the relationship between their abundance and meso-habitat availability. The output of the meso-habitat methodology was the relationship among meso-habitat availability and the range of modeled flows. Species Used for rite Biological Analysis The primary species used in the instream flow analysis were the bluehead sucker (Catostonllls discobollls), flannelmouth sucker (Catostomlls latipinnis) and roundtail chub (Gila robllsta). These species, quite common in the Colorado River, comprised about 80 percent of fish sampled between Rifle and Palisade. Anderson (1977) speculated that fishery abundance was likely at carrying capacity set by the physical habitat. Samples made by the Recovery Program (ISMP) in the Colorado River downstream of Grand Junction to Loma found native fish were about 90% of the catch (Elm bald 2003). Osmundson (1999) reported high catch rates of native suckers in the Colorado River, with the highest catch rate in the IS-Mile Reach. The thriving populations of these three native species suggest that habitat, base flow alternations or non-native introductions have not negatively impacted their life history requirements in the river. In fact, studies suggest the Colorado River is an excellent location to study dynamics between these three native fish and physical habitat.