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Hann 1989). Such output is nonsense because the ecological data for these fishes clearly shows the importance of backwaters and eddies that occur at much higher flows. The problem here is that IFlM probably should never have been used in the big river reaches of the Upper Colorado River Basin. When low velocity habitats are abundant, as they are throughout the potamon of the Colorado River system, the simulator underestimates the WUA; in fact, the model cannot deal with zero flow habitats. This explains why IFIM works well only in small streams where the channel is characterized by unifonnly varying flow (e.g., the low velocity profIle reflects steady, uniform flow which is also an assumption of the HEC-2 hydrology simulator that is often used in IFIM, my observations). Also, habitat suitability curves were probably biased because the fish were difficult to observe or collect in the usually turbid, deep water of the Yampa and Green Rivers (Rose and Hann 1989) which is precisely why the adult monitoring program (U.S. Fish & Wildlife Service 1987b) emphasizes shallow, shoreline habitats that can be effectively sampled by electrofishing. However, the fishes routinely use deep water habitats (e.g., Tyus and McAda 1984, McAda and Kaeding 1991), and movement between habitats (e.g., channel, backwaters) on a diel basis cannot be accounted for in the method. A fmal point to keep in mind is that the utility of IFIM evolved a great deal during the period that data were being gathered in the Upper Colorado River Basin studies, and deficiencies in the method with regard to the Colorado River perhaps were not apparent at the time much of the data were gathered. Are There Other Options? Certainly strong inferences can be derived from careful measures of channel processes that influence habitats important to the fishes. Reiser et al. (1989b) recently described the physical relationships between hydraulics and movement of sediments with respect to deriving flushing flows to remove fme sediments entrained within the bottom of an alluvial river, as described above for the Gunnison River. These principles of flow mechanics can be used to derive other formalized approaches to manage flows for the purpose of maintaining channel forms the fishes use. Sediment transport mechanics depend upon detailed information on sediment gradation, channel 50 " ~