Assessment of Flow-Ecology Relationships for Determining Environmental Instream Flow Standards: A Meta-Analysis Focused on the Southeastern United States

Rivers provide valuable ecosystem services that are increasingly threatened by population growth, climate change, land-use change, and dams, all leading to alterations of natural flow regimes. A major management and policy tool that can help to protect the ecosystem services sustained by rivers is environmental instream flow standards, or ranges of flows that must be left in rivers for environmental purposes, notably for aquatic and riparian habitat. Although environmental instream flow requirements can be assessed using a variety of methods, most of these methods require implicitly or explicitly establishing relationships between flow and habitat of species of concern. In recent years, the number of such flow-habitat studies completed for U.S. states or interstate river basins has grown, in part because of the increasing use of the Ecological Limits of Hydrologic Alteration (ELOHA) framework to develop regional environmental instream flow standards. These flow-ecology studies are, however, expensive and time-consuming to conduct, given the large amounts of hydrological and ecological data and modeling required. The ability to generalize the results of past flow-ecology studies to larger areas would be helpful in developing regional environmental instream flow standards that are ecologically effective. Here, we conducted a meta-analysis of past ELOHA-associated flow-ecology studies in the Southeastern United States, a region that leads the nation in freshwater biodiversity and is relatively similar across states in terms of climate and physiographic regions. The meta-analysis included the states of Virginia and North Carolina and the Potomac and Upper Tennessee interstate river basins. For each state or basin, we used the published data to determine the flow metrics that resulted in the greatest changes in ecological metrics, and the ecological metrics that were most sensitive to hydrologic alteration. The flow metrics that were most important in preserving ecological metrics were identified as potential targets for setting environmental instream flow standards in Southeastern U.S. states that currently lack such standards. The meta-analysis contributes to understanding of the physical controls on aquatic and riparian habitat and potentially to improved water management and policy.