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25 collected either at several different discharges covering the entire range of conditions to be analyzed or at one discharge and then used to calibrate a simulation model which can predict conditions at other 1 discharges. The second approach using hydraulic simulation requires significantly less field work to construct a habitat-discharge curve (see Sect. 2.4.3). Other regional differences are associated with the specific attributes that are used to represent instream habitat conditions. 2.4.1 Habitat-Discharge Curves Several physical parameters were suggested as the flow-dependent variable to be used in developing habitat-discharge relationships (Bartschi 1976). These attributes can include such hydraulic parameters as stream surface width, wetted perimeter, average water velocity, maximum depth, or cross-sectional area at each transect. Presentation of the data is then made in a graphic format with discharge represented on the "x" axis and the habitat parameter on the "y" axis. For example, in situations where a fisheries biologist is concerned about instream flow needs for fish growth and rearing, wetted perimeter might be chosen to represent habitat condition, under the assumption that food production is proportional to bottom surface area. The habitat-discharge relationship can then be represented as relative changes from some reference flow (Fig. 2-2a) or in absolute terms (Fig. 2-2b). Two different approaches were used to derive instream flow recommendations from the habitat response curves. The minimum discharge can be set at the flow which produces a fixed percentage reduction from a reference flow in a particular habitat attribute (e.g., 20% reduction in wetted perimeter is maximum degradation allowable; Bartschi 1976). This approach is called a habitat retention criterion. The second approach, which is also included in several other assessment methods, relies on determination of the "inflection point" on a habitat-discharge response curve (e.g., point