Hydropower dams play a critical role in the health of river ecosystems throughout the United States, and hundreds of these dams will be relicensed by the Federal Energy Regulatory Commission (FERC) in the coming years. Such licenses lock in the operating and environmental protection requirements of such dams for periods of up to 50 years. Given the complex, dynamic nature of river ecosystems, as well as the impacts of climate change, there is pervasive scientific uncertainty about how to best manage dams for power production while protecting and enhancing environmental values such as water quality and fisheries. Unless dams are managed adaptively, with licenses that provide pathways for gathering and applying new knowledge and responding to changing conditions, we run the risk of locking in mistaken approaches and stymieing environmental improvements on our rivers for the next half century.

Water managers face tough challenges in sustaining the health
and availability of rivers while meeting increasing demands for
their use. One tool that can give hydro project owners guidance
is a six-step framework for ecologically sustainable water management
developed by The Nature Conservancy.

Systems for management of water throughout the developed world have been designed and operated under the assumption of stationarity. Stationarity-the idea that natural systems fluctuate within an unchanging envelope of variability-is a foundational concept that permeates trainingand practice in water-resource engineering. It implies that any variable (e.g., annual streamflow or annual flood peak) has a time-invariant(or 1-year-periodic) probability density function (pdf), whose properties can be estimated from the instrument record. Under stationarity, pdf estimation errors are acknowledged, but have been assumed to be reducible by additional observations, more efficient estimators, or regional or paleohydrologic data. The pdfs, in turn, are used to evaluateand manage risks to water supplies, waterworks, and floodplains; annual global investment in water infrastructure exceeds U.S.$500 billion.