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INSTREAM FLOW INCREMENTAL METHODOLOGY (IFIM) e The U.S. Fish and Wildlife Service's Instream Flow Incremental Methodology (IFIM), as outlined by Bovee (1982), is a set of ideas used to assess instream flow problems. The Physical Habitat Simulation System (PHABSIM), described by Milhous et al. (1981), is one component of IFIM that can be used by inves- tigators interested in determining the amount of available instream habitat for a fish species as a function of streamflow. The output generated by PHABSIM can be used for several IFIM habitat display and interpretation techniques, including: 1. Optimization. Determination of monthly flows that minimize habitat reductions for species and life stages of interest; 2. Habitat Time Series. Determination of the impact of a project on habitat by imposing project operation curves over historical flow records and integrating the difference between the curves; and 3. Effective Habitat Time Series. Calculation of the habitat require- ments of each life stage of a fish species at a given time by using habitat ratios (relative spatial requirements of various life stages). Suitability Index Graphs as Used in IFIM PHABSIM utilizes Suitability Index graphs (51 curves) that describe the instream suitability of the habitat variables most closely related to stream hydraulics and channel structure (velocity, depth, substrate, temperature, and cover) for each major life stage of a given fish species (spawning, egg incuba- t ion, fry, juvenil e, and adul t). The specifi c curves requi red for a PHABSIM analysis represent the hydraulic-related parameters for which a species or 1 ife stage demonstrates a strong preference (i. e., a speci es that only shows preferences for velocity and temperature wi 11 have very broad curves for depth, substrate, and cover). e WELUT has standardized the terminology pertaining to SI curves and designated four categories of curves. All species curves for HEP and IFIM are referred to collectively as sui tabil i ty index (SI) curves or graphs. The designation of a curve as belonging to a particular category does not imply that there are differences in the quality or accuracy of curves among the four categories. Category one curves are the most common type presently available for use with HEP or IFIM. Usually category one curves have as their basis one or more literature sources. Some SI curves may be derived from general statements made in the literature about fishes (i.e., rainbow trout spawn in gravel; fry prefer shallow water). Some category one curves may come from 1 iterature sources which include variable amounts of field data (i.e., from a sample size of 300, fry were observed in vel oci ties rangi ng 0.0 to 3.0 ft/sec, and 80% were found in velocities less than 1.0 ft/sec). Other category one curves may be based entirely on professional opinion, by using the Delphi technique or 38 e