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Site Characterization Length, mean width and wetted area for each simulated flow were determined for each site using the two-dimensional modeling output data. Station lengths were measured along the channel thalweg. Widths were determined by dividing the wetted channel area at 600cfs (100cfs for Big Gypsum) by the station length. Longitudinal profiles were generated in ArcInfo by sampling channel bathymetry at two-meter intervals along the channel thalweg. HABITAT QUANTIFICATION A primary objective of this study was to objectively quantify changes in meso- habitat composition over a range of flows at each site. Meso-habitat is defined at the reach level and includes pools, runs, riffles and rapids. Pools have low velocity, runs have moderate velocity, riffles are swift areas and rapids have the fastest current. For modeling purposes pools were given a velocity of zero to 0.15 misec, runs ranged from 0.15 to 0.6 m/sec, riffles had velocity ranging from 0.6 to 1.5 m/sec and rapids had velocities over 1.5 m/sec (Table 4). Habitat suitability for fish was also a function of depth. Very shallow areas were of high value to small fish, but they had no habitat value to large fish. Pools and runs had five depth categories, riffles had four and two depths categories for rapids (Table 4). Using only depth and velocity, we defined 16 different non-overlapping meso- habitat types for use in this study. These 16meso-habitat types provided a general representation of habitat diversity that was inclusive for a fish community composed of a variety of size-classes and species. These 16 meso-habitat types were then mapped using data provided by the 2-D flow models. In each case, solution files (2-D model output) were imported into ArcInfo and then linearly interpolated into 1 x 1-meter grids representing depth and velocity for a given flow. Depth and velocity grids were then sampled to create 1x1-meter meso-habitat grids. The abundance of meso-habitat was determined by counting the number of grids of each meso-habitat type. Surface maps of meso-habitat were created either in ArcView or by importing 2-D plotfiles into SMS (the 2-D modeling package by Boss International). DEVELOPMENT OF MESO-HABITAT SUITABILITY Meso-habitat suitability was developed using the fish sampling data. The electro- fishing data was summarized for individual electro-fishing sub-reaches (polygons). The total density and biomass calculated over the entire station did not indicate how fish were distributed within a station, whereas the electro-fishing sub-reaches provided much more detail on fish distribution within the stations. Electro-fishing- sub-reaches had the same starting and end locations between passes and years and were digitized on the aerial photos of the study stations using ArcView. The sum of the area and fish from all electro-fishing sub-reaches equaled 100% of the entire station. 31