Abstract
Although most rivers in the US are regulated to support human water uses, while preserving aquatic ecosystems, the effectiveness of regulations is rarely assessed. Concerns for restoring natural flow regimes have increased requirements for "run-of-river" (ROR) operation. It is widely believed that such restrictions (1) are mandated to protect aquatic biota, (2) decrease hydropower generation per unit flow, and (3) decrease energy revenue. In this study, we tested these three assumptions by reviewing hydropower projects that changed from peaking to ROR operation as a result of license modifications. ROR was often prescribed in states with strong water quality certification requirements and migratory fish species. Although benefits to aquatic resources were frequently cited, changes were often motivated by other considerations. When testing the second assumption, we found that the overall change in annual generation across projects was not significant, and much smaller than the effect of climate. However, we detected a significant decrease in generation efficiency at one-quarter of hydropower projects. When testing the third assumption, we found the expected decrease in flow during peak demand at seven of ten projects. At the remaining projects, diurnal fluctuations actually increased due to operation of upstream storage projects. We discuss the economic implications of these results, including both producer costs and ecological benefits. Regional- or national-scale studies of hydropower regulation are long overdue. Public dissemination of flow data, license provisions, and monitoring data would facilitate regional scale studies like this one, while increasing regulatory transparency and providing feedback to decision makers.
