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Riffle Productivity Invertebrate production in riffies is greater than other riverine habitats (Brown and Brussock 1991). In the upper Colorado River basin, riffies in steeper river reaches are capable of supporting very productive benthic food webs, and these food webs are more stable, complex, and productive in upstream reaches associated with cobble substrate (Stanford 1994). Data from the Upper Colorado River suggest that primary and secondary production is greatest in the upstream, higher gradient reaches with more riffies (Lamarra 1999), which also coincides with highest fish densities (Osmundson 1999). Anderson and Irving (1999) pointed out that physical conditions that maintain riffies should be preserved, because a strong relationship between stable and predictable environment and stability and integrity of the aquatic community, is well supported in the literature (Allan 1995, Brown and Brussock 1991, and Brusven et al. 1990). Flows between 400 and 500 cfs cover 95% of available surface area for most riffles, and this seems to us adequate to provide for near maximum riflle production during the base flow period. We used two criteria for defining minimum riflle needs for fish. First, we examined riftle surface area vs. discharge and found, not surprisingly, the same linear relationship as with night foraging habitat. And second, we examined wetted perimeter vs. discharge and found a curve break suggesting more rapid loss in riftle habitat at flows < 161 cfs. It is our opinion that the wetted perimeter method is moreinfonnative. As stated in the Model Critique section, the physical habitat simulations of habitat areas produced approximations but are not precise. Wetted perimeter estimates, on the other hand, required fewer assumptions and are more precise. Similar curve break flows were found for depth, width, and velocity. That these hydraulic parameters 16