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27 'I' in Fig. 2-2b). The interpretation of the inflection point on a habitat-discharge curve is somewhat different from the formal definition; it is defined simply as the point where a major change in slope occurs. This approach represents an attempt to use the rate of change in a physical habitat attribute (t.g., meters of wetted perimeter per unit flow) to identify a threshold below which conditions are most rapidly degraded (see Sect. 3.4). In addition to methods that measure single attributes of habitat condition, U.S. Forest Service personnel developed aggregative indices of habitat quality that incorporate multiple aspects of the aquatic environment (Dunham and Collotzi 1975, Cooper 1976). These procedures are also oriented toward transect-type data sets, but rely on the assignment of nonparametric ratings to "quantify" subjective evaluations by field personnel. Each of four components of habitat, pool measure (percentage of stream width with pool-type habitat), pool structure (quality of pool habitat based on depth, cover, etc.), substrate type, and riparian environment, is rated by assignment of an integer value between 1 and 4. The average of these four variables represents the habitat value index. This procedure is repeated for several different flow conditions, and a graph similar to that shown in Fig. 2-2a is constructed. The instream flow requirement is the discharge that results in 80% of the habitat index at a bank-full reference flow. 2.4.2 Simplified Staff-Gage Analysis Efforts to develop a relatively simple field method for quantifying the relationship between habitat condition and stream flow led to what can be called staff-gage analyses. This approach is based on a stage-discharge curve similar to that used at USGS stream gaging stations, but a slightly different emphasis is placed on transect location. In contrast to the USGS hydraulic criteria for selecting a transect site, ecological criteria are used, preferably by a qualified fisheries biologist, to identify critical habitat for a target species