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.' . . discharge. Easy to read and more detailed descriptions of IFIM are given by Gore and Nestler (1988) and Nestler et al. (1989). This procedure has been widely used to justify flow provisions in regulated streams throughout North America, in some cases leading to state statutes to guarantee protection of aquatic biota (Reiser et al. 1989a). Even though IFIM has become an industry standard (Reiser et al. 1989a), it has a number of faults that are not widely recognized or understood within the management circles. Concern exists with respect to use of suitability curves as probability functions (Patten 1979, Mathur et al. 1985. Moyle and Baltz 1985). the assumption of independence of depth, velocity and substratum (patten 1979. Malthur et ale 1985), the lack of a demonstrated relationship between WUA and a meaningful measure of productivity or biomass (Mathur et al. 1985. Bowlby and Roff 1986, Conder and Annear 1987, Scott and Shrivelll987) and lack of any relationship with regard to many other ecosystem processes, such as predation and other density-dependent relationships, that clearly influence population structure (Moyle and Baltz 1985, Bowlby and Roff 1986, Orth 1987, Stanford and Ward 1992a). To my knowledge none of these criticisms have been resolved, nor is it likely they will be. However, these criticisms have been placed in perspective with respect to the rationale and intent of the IFIM, which is often misunderstood, misrepresented and misused (Gore and Nestler 1988). For example, the model was not intended to predict biomass. It is a physical habitat simulator. Even when the model is applied properly, a variety of problems may emerge depending on input choices, which necessitates a clear understanding of how the model works. The simulator can use a variety of hydraulic predictors (e.g., the HEC-2 flow model of the U.S. Army Corps of Engineers), each of which has biases and therefore will result in different WUA calculations (Gan and McMahon 1990). Suitability curves not derived on site (i.e., curves given in the literature) are often used, which can also bias output (Gore and Nestler 1988). The IFIM was used in an attempt to derive flow recommendations for specific river segments of the Upper Colorado River Basin with respect to the endangered fishes. However, in the analysis WUA often was maximized for various life history stages of squawfish and humpback chub at very low flows that in the historical record were exceeded most or all of the time (Rose and 49 .