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<br />condition for any desired flow regime. The hydraulic model is calibrated to <br />reproduce water surface elevations and horizontal velocity distributions <br />observed at selective stream flow conditions. The IFG's simulation models <br />normally use stream channel geometry and velocity data from several cross <br />sections within a relatively short stream reach, Each transect can be sub- <br />divided into as many as 100 cells (conveyance areas) to facilitate detailed <br />analysis of the spacial distribution of depth and velocity combinations. Once <br />properly calibrated, the computer program will calculate the water surface <br />evaluation and respective horizontal velocity distribution at each transect <br />for all desired discharges. The simulated water service elevations and <br />velocities are then passed from the hydraulic model to IFG's HABTAT model <br />(Main 1978a), <br />Within the HABTAT model, the mean depth of each cell is computed by <br />subtracting stream bed elevations from the simulated water surface elevation. <br />Surface areas associated with the occurrence of various combinations of <br />depth/velocity values are calculated by multiplying the width of the cell by <br />the sum of half the reach distance to the next upstream and the next down- <br />stream transects. This procedure is illustrated in Figure 2. <br />The stream reach simulation takes the form of a multi-dimensional matrix <br />showing the surface area of cells having various combinations of physical <br />habitat characteristics (i.e., depth, velocity, substrate, and cover when <br />applicable). Table 1 illustrates a depth-velocity matrix. The number in the <br />upper lefthand corner of the matrix refers to 195 square feet of surface area <br />per 500 feet of stream length having a combination of depths less than 1.0 <br />feet and velocities less than 0,5 ft./sec, This represents the summation of <br />the surface areas of all the individual cells within the simulated reach with <br />that combination of depth and velocity, This 195 square feet of surface area <br />is not necessarily contiguous, <br /> <br />31 <br />