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2 BIOLOGICAL REPORT 24 strongly supported reregulation of flows in the Green and Colorado rivers to produce more natural, seasonal patterns. Based on review of the ecological information and recognizing the problems in the methodological approaches that were used to derive flow recommendations, several key uncertainties seem to be critical to the goal of establishing flow regimes that will ultimately recover the endangered fishes. • Flow seasonality and its correlates (e.g., temperature and physical habitat) may not be the factors limiting recovery of the native fishes. • Given the high societal value placed on tailwater trout fisheries and the high priority placed on meeting entitlements under the Colorado River Compact and current water law, water volume in the Colorado and Green rivers may be insufficient to produce flows required to recover the fishes. • Channel and floodplain morphology in time and space is not a simple flow-area relationship, and complex interactions not yet fully,understood may emerge that will compromise recovery of the fish. • What is the tradeoff between the propensity of endangered fish larvae to drift downstream and the need for high flows to maintain connectivity between the channel and backwaters and wetlands? • Can food webs reestablish in key low velocity habitats (backwaters) to the extent needed to recover the fishes, given the windows permitted or needed for hydropower operations? • Can the endangered fishes expand their range and productivity, given the downstream extension of coldwater environments caused by regulation, and is the locality of the transition zone between cold and warm reaches likely to stay constant as reregulated flow regimes are implemented? • Interactions with nonnative fishes may limit recovery of endangered fishes regardless of flow provisions. The report concludes with recommendations that couple management action (implementation of interim flow regimes) with additional study to resolve the uncertainties presented above. The recommendations reflect an ecosystem approach to resolution of flows needed to protect and enhance the endangered fishes of the Upper Colorado River Basin. In essence, these recommendations constitute a new, holistic instream flow methodology. • Implement interim flows that reestablish seasonality, with spring peaks that approach the amplitude and frequency of preregulation events, and summer and winter baseflows with daily changes (not daily volume) limited to near preregulation conditions (probably no more than about 5% per day). • Provide common understanding of water availability so that interim flows can be provided in relation to precipitation and legal flow abstraction in each subbasin. • Improve the standardized monitoring program as a mechanism to evaluate effectiveness of interim flows by adding a community ecology perspective. • Diversify the research program to resolve critical uncertainties associated with interim flows. • Implement a peer review process to ensure that research and monitoring objectives are based on solid science and are responsive to the need to resolve uncertainties associated with the interim flows. • Implement a management process that can adaptively change the interim flows as new implications from monitoring and research are forthcoming. The recommended methodology needs unambiguous endorsement to be successful. Success or failure will be judged by long-term trends in the populations of the endangered fishes.