ceptable risk. In addition, when a species at the time of listing occupies a broader range than appears to be required to maintain a minimal viable population, critical habitat designation must include decisions about which areas to protect and which to allow to be destroyed. Those choices include scientific considerations, to the extent that some areas appear more important than others to the species, but also require balancing of equities with respect to competing human uses.

The uncertainty facing Platte River managers is high but not unusual for river systems in the United States. Furthermore, uncertainty about the Platte River is an element of all management decisions, not just decisions that restrict diversions. Just as there is uncertainty about stream-flow requirements of the listed species in the central Platte, there is uncertainty about the efficacy of the tradeoffs authorized by USFWS in its recent biological opinions authorizing new diversions in return for habitat-restoration efforts or payment of mitigation fees into a habitat acquisition and restoration fund.

Adaptive management is one approach for identifying and taking steps to close key information gaps. Adaptive management is being implemented, at least in small ways, in some other river systems. Platte River management might benefit from more-systematic efforts to accumulate data about the effects of flow levels on the physical and biological system and to incorporate the data into future management decisions.

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