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These criteria2 were prepared by life history stage for those streamflow parameters directly influenced by changes in flow regime or channel geometry and which were considered to most directly affect fish distribution; depth, velocity, substrate and temperature. Species criteria for the Salmonid fishes were developed and distributed by the Instream Flow Group in 1978 (Bovee 1978) , The habitat suitability curves used in conjunction with the IFG metho- dology are based on the understanding that individuals of a species tend to select the most favorable conditions available within a stream for habitation, bu~ will use less favorable conditions with less frequency eventually leaving an area if possible before conditions become lethal. Subsequently individuals would be most frequently observed (sampled) in nature inhabiting their most preferred habitat conditions, Implicit in the use of these criteria is the assumption that frequency of observation is, in fact, indicative of habitat preference and the understanding that the data base used to construct the curves was obtained in an unbiased manner. Figure 3 presents example criteria for adult smallmouth bass, For a given parameter value a weighting factor may be determined directly from the curve. For example, a depth of 2,4 feet and a velocity of 0,6 ft./sec, yield respective weighting factors of 0,37 and 0.80. The composite weighting factor (C) for a cell with the depth of 3,5 feet and a velocity of 0.5 by adult small mouth bass is (0.37 x 0.80) or 0.3. ~ Editors note: Prior to the workshop being reported on in this proceedings these species criteria curves were referred to as probability-of-use curves, However, it became apparent during the course of the workshop discussions that these curves needed to be renamed. Several names have been considered but the one chosen for use is "Habitat Parameter Suitability" curves. 34