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The stream reach simulation utilized by IFG uses several cross- sectional transects, each of which is subdivided into 9 to 20 subsec- tions. The computer program then treats each subsection as an essen- tially separate channel. The mean depth and velocity of each subsection may then be calculated for any stage (water surface elevation). An area represented by these values of depth and velocity is cal- culated by multiplying the width of the subsection by half the distance to the next transect upstream and downstream (Figure 9). The output of the stream-reach simulation is in the form of a multidimensional matrix showing the surface area of stream having different combinations of hydraulic parameters (i.e., depth, velocity, substrate, and cover when applicable). Table 5 illustrates a depth- velocity matrix, although the analysis is not limited to two dimensions. The outlined numeral in the-upper left-hand corner of the first matrix refers to 195 square feet per 100 feet of stream having a combination of depths less than 1.0 feet and velocities less than 0.5 ft/sec. This is the total summation of areas within the stream reach with that combin- ation of depths and velocities. These areas are not necessarily con- tiguous. COMPOSITE PROBABILITY OF USE The composite use probability of any combination of hydraulic con- ditions encountered in a study reach may be determined from the indi- vidual probability of use curves for each species and life stage. r Figure 10 illustrates probability of use curves for adult small- mouth bass for depths and velocities. For a given increment of each parameter the use probability is read directly from the curve. For example, the use probability for the depth increment of 3.5 feet is 0.37. The use probability for the velocity increment of 0.5 ft/sec is 0.81. The composite use probability for adult smallmouth bass for a depth of 3.5 feet and a velocity of 0.5 ft/sec is 0.30 (0.37 X 0.81). A composite probability is similarly calculated for each stream reach sub- section. Substrate or cover may also be incorporated into this determination of composite probability following the procedure detailed above. In the preceeding example, the composite probability for a stated combination of depth and velocity was found to be 0.30. If the substrate found with that combination of depth and velocity had a probability of use of 0.90, then the composite probability of use for that combination of depth, velocity, and substrate would be 0.27 (0.37 X 0.81 X 0.90). 30