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36 2.7 Preferred Area Two methods that quantify physical habitat in terms of preferred areas have contributed significantly to the development of instream flow assessment. Both increase the dimensionality of analysis compared to the usable width approaches by using multiple transect data and measuring habitat in units of area rather than of width. The Washington Department of Fisheries, in cooperation with the USGS, applied a two-dimensional mapping technique, referred to as the Washington Method, to quantify the area of streambed available for salmon spawning (Collings 1972, 1974). Biological criteria for spawning consist of upper and lower bounds on preferred depths and velocities. Depth and velocity measurements are taken across four transects at a potential spawning site, and isopieth maps of the streambed are constructed to show the distribution of hydraulic parameters at a fixed discharge (Fig. 2-4). Areas satisfying both the velocity and depth criteria are designated as spawnable portions of the stream. This procedure is repeated for a minimum of five different stream flow conditions to develop a response curve of spawnable area vs discharge over the range of flows of interest. No hydraulic modeling is used in predicting depths or velocities. The preferred spawning flow is defined as the flow with the maximum spawnable area. Instream flow requirements are set at the flow which maintains 75% of the maximum spawning area. This type of flow recommendation is another example of a habitat retention criterion. A second approach to quantifying instream habitat on an areal basis was developed by fisheries biologists of the Pacific Gas and Electric Company (Waters 1976). This work, which was initiated in the 1950's in California, predates all of the methods discussed previously. Like the Washington Method, two-dimensional maps of the stream environment are developed from transect data, but continuous weighting criteria for velocity and depth are used to calculate habitat values. Again, no hydraulic simulation modeling is used to