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<br />compatible with the theoretical basis of the models (i.e, steady flow within a <br />rigid boundary), This method is composed of four basic components: (1) field <br />measurement of stream channel characteristics using a multiple transect <br />approach; (2) hydraulic simulation to determine the spacial distribution of <br />combinations of depths and velocities with respect to substrate and cover <br />objects under alternative flow regimes; (3) application of habitat suitability <br />criteria to determine weighting factors; and (4) calculation of weighted <br />usable area (gross habitat index) for the simulated stream flows based on <br />physical characteristics of the stream, <br />Four primary variables can be identified which determine the character of <br />instream habitat conditions: (1) water chemistry; (2) food web relations; (3) <br />flow regime; and (4) channel structure. Associated with each of these major <br />variables are the respective subsets of variables which interact to provide <br />the myriad of physical-chemical conditions to which the stream biota respond. <br />These four primary variables also offer a logical division for approaching the <br />task of quantifying the effects of land and water management decisions on <br />instream fishery resources. <br />During the 18 months preceding this workshop the Instream Flow Group's <br />efforts concentrated on describing cause-effect relationships between stream <br />flow alterations and instream fishery habitat potential. In western streams <br />the most direct relationships (habitat constraints) are attributable to flow <br />regime and/or channel structure, Consequently, hydraulic simulation modeling <br />is of central importance to the incremental methodology, <br /> <br />- <br /> <br />27 <br />