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Therefore, the representativeness of this one measured side channel is unknown. The National Ecology Group recommended a study to determine the relationship between the flow regime and other pertinent physical aspects, such as sediment-to-bar formation. This would identify the type of bars that form, their distribution and frequency, and if a flow recommendation is necessary for them to continue to form as presently occurring or if a different regime would result in an improvement, e.g., a type of bar that provides more habitat, persists longer, or allows backwaters to exist within a wider range of flows (NEC, 1987). ' They further recommended a time series analysis to det~ct limiting habitat events such as low surface area of important backwaters. Some items to consider include the dynamics of side channel backwaters and the biological aspects of how they are used by species and the physical conditions necessary to make them suitable. For example, is a side channel unusable when the flow drops and leaves the habitat as an isolated pool? Or does a temporary (how long?) isolated pool serve as a barrier to predators? How long does water quality remain suitable in isolated pools? Other techniques, such as standard aerial or video photography, may more readily lend themselves to an accurate assessment of backwater habitat availability as a function of discharge; particularly when looking at several different locations and/or extended river reaches many miles long. CONCLUSIONS PHABSIM applications on large, turbid, hydrologically complex rivers remains problematic. As Rose and Hann (1989) pOinted out, the combination of hydraulic data collected along stream cross-sectional transects, and SI curves assumes that suitable habitat for fish species can be accurately described by three microhabitat variables (depth, velocity and substrate), when macrohabitat variables, such as channel stability, temperature and water quality, are assumed suitable throughout a given range of flows. Accepting the predictive results of PHABSIM requires the assumption that fish population response has some correlation to changes in WUA. This assumption has never been validated for large, turbid rivers. It has not been demonstrated that WUA, as defined by depth, velocity, and substrate has any relationship to endangered fish populations whatsoever. Determining the degree of accuracy of existing endangered fish SI curves and the corresponding predictive reliability of PHABSIM, and what it means to the overall health of fish populations will require many years of extensive field validation and hypotheses testing. Until species criteria can be validated, they are of limited value. Over the past ten years PHABSIM has been applied extensively throughout the UCRB and with very rare exception has not produced satisfactory results when applied to endangered fishes. The link between the physical model and the biological needs of endangered fishes needs substantial improvement, testing and validation before it can be used as a basis for determining instream flow requirements. 38