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INSTREAM FLOWS TO ASSIST THE RECOVERY OF ENDANGERED FISHES 29 Are There Other Options? Strong inferences can be derived from careful measures of channel processes that influence habi- tats important to the fishes. Reiser et al. (1989b) described the physical relationships between hy- draulics and movement of sediments with respect to deriving flushing flows to remove fine sediments entrained within the bottom of an alluvial 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 de- pend on detailed information on sediment grada- tion, channel geomorphology, and channel slope. If data needed to calculate sediment mass balance are available and are coupled with detailed topographic information, derived either from aerial photo- graphs or surveys over the period before and after regulation, the morphological dynamics of the channel can be documented (cf., Andrews 1986; Lyons and Pucherelli 1992), and informed ap- proaches to flow negotiations can proceed. How- ever, regime analyses too often rely on untested assumptions that some flow volume and rate rela- tionship, usually bankfull flow, is the dominant channel-forming flow. Determination of bankfull flow is problematic owing to local variations in channel morphology coupled with usually too few data on hydraulics of the reach during peak flow events. In my view the preferred approach is a thorough, empirical understanding of sediment gradation, channel geomorphology, and channel slope, with which movement of sediment and hence the dy- namics of many physical habitats important to aquatic biota can be estimated as a function of the amplitude of peak flow events. Andrews and Nelson (1989) used this approach to document topographic responses of a large bar complex in the Green River over a history of flow events. A major advantage of the model is that, although it is deterministic, flows, sediment supply, and, to some extent, topog- raphy can be stochastic. The model is being used to predict dynamics of sediment transport and chan- nel topography in response to flow variation else- where in the Colorado River system. Model devel- opment and verification is greatly assisted by recent improvements in automated field surveying equipment (total stations) that allow rapid and very accurate measurements of local topography (E. D. Andrews, personal communication). However, as concluded by Reiser et al. (1989b), the most certain method to determine relationships between peak flow events and channel features in a regulated river is to tag an array of bed materials, carefully survey channel topography (sensu Andrews and Nelson 1989), and relate movement of materials and changes in topography to different flow events carefully controlled by reservoir releases. However, the flow peaks have to be high enough to move the tagged bed materials, which can be approximated using standard hydraulic calculations. From a more biological perspective, several al- ternative approaches are possible. Binns and Eis- erman (1979) predicted trout biomass in Wyoming streams with a habitat quality index (HQI) in which 11 habitat variables, including baseflow and an- nual change in discharge, thought to influence trout populations were rated subjectively. The pre- dictions were significantly correlated with actual measures of biomass. The Delphi rating schedules used in this technique apparently resolved much of the nonlinearity usually observed in relationships between habitat descriptors and fish biomass. The Delphi method is an iterative procedure for obtain- ing consensus of best professional judgment, when direct measurements are not available (Zuboy 1981). However, Bowlby and Rolf (1986) were not as successful in using the method in Ontario streams because trout density changed within stream segments when habitat variables remained the same. Other biophysical indices of habitat qual- ity have been proposed (cf., Osborne et al. 1992; Rabeni and Jacobson 1993); they have been used to establish relative influences of stream regulation in different streams, but to my knowledge they have not been used to examine incremental effects of now. A general (simple application in different streams) incremental flow-biomass model that is statistically precise (repeatable) and accurate (de- scribes reality) is probably not attainable, espe- cially in large rivers like the upper Colorado, where ecosystem structure and function are complex and poorly known. However, the problem can be ap- proached from a multidisciplinary perspective, where strong inferences about how the endangered fishes are likely to respond to reregulated flow regimes can be derived from process-oriented stud- ies that demonstrate key biophysical relationships. Linking hydrology, geomorphology, and limnology in an ecosystem context is the key (Stanford and Ward 1992a), and I recommend below a new ap- proach for reaching an ecosystem level of under- standing with respect to flow provision in potamon reaches of the Upper Colorado River Basin.