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hydraulic simulation; calculation of joint depth, velocity, substrate composi- tion, and vegetative cover distributions; determination of probability of use at all possible combinations of these parameters; and determination of the weighted usable area for the instream activity of interest (see Bovee and Cochnauer, 1977 and Hyra, 1978). Hydraulic simulation is a mathematical description of the physical characteristics of a stream reach. . . , In the simplest of examples, a series of measurements of depth, velocity, and width across a crosssec tion (transec t) describes the spatial distribution of depths and ve loc ities wi thin the crosssec tion at the discharge (flow) which was measured. Repetition of the process at many dis- charges {"ould result in a different des- cription of these distributions at each discharge measured. Extension of the process to several crosssections ... allows the description of such distributions for a stream reach (Milhous and Bovee, 1978). The simulation allows estimation of the stream surface area for each possible combination of the hydraulic parameters. Probability-of-use curves derived from field observations or individual capture locations indicate the relation between hydraulic parameters and frequency of use. For example, juvenile cutthroat trout range in depths from 0.5-4 ft. Outside this range, probability of use is zero. The depth with the greatest frequency of observation, 1.5 ft, is assigned a probability of use of 1. O. Mul tiplying the probability of use for separate parameters yields the joint probability of use. Thus, the probability of use for juvenile cutthroat trout at a depth of 1.5 ft, a velocity of 2 ftls, a temperature of 400F, and a substrate composed of gravel is 0.15 0.0 x 0.4 x 0.6 x 0.6 = 0.144) (Bovee, 1978) . Multiplication of the areas by the joint probabilities of use and summa- tion yields the "weighted usable area" for the activity of interest. In this scheme, 100 ft2 of stream surface with a probability of use of 1.0 are equivalent to 500 ft2 with a probability of use of 0.2. The final task is projecting changes arising from new storage reservoirs or depletions. Obviously this requires detailed information concerning natural flow condi- tions (such as mean monthly discharge), existing and projected consumption patterns, location of diversion and return flow canals, and reservoir opera- ting policies. Though the Incremental Methodology represents the "state of the art" for streamflow evaluation, there are other competing methodologies with different time and resource requirements. As a successor to the stream classification project, the US Fish and wildlife Service has current contracts with 12 Western States to evaluate several of these methodologies under a variety of stream morphologies and habitats and to quantify streamflows necessary to protect the most critical fisheries.S These activities offer great promise 5personal communication, Jeff Johnson, US F&WS, Denver, Colorado, May, 1979. 4