Changing hydrologic volatility and associated risk to freshwater biota across the conterminous United States, 1980–2020

Hydrologic volatility—the pronounced variability in flow anomalies relative to natural expectations over sub-annual timescales—is of increasing concern to water management and freshwater ecosystems, yet research quantifying the effects of volatility on freshwater biota is lacking across large spatial scales. In this study, we use a causal inference framework to quantify responses of freshwater biota ( i.e., fishes and benthic macroinvertebrates) across regions of the conterminous United States (CONUS) to a gradient of monthly-scale hydrologic volatility in the 12-months prior to sampling. We then examine trends in hydrologic seasonal volatility and predict volatility-driven impacts to freshwater biota at over 3,300 locations across CONUS for water years 1980–2020. Our results broadly show that increasing hydrologic volatility resulted in increased risk of poor biotic condition indices in many regions of CONUS. Spatial clusters in the trends in volatility and volatility-driven risk to biotic conditions indicate variable patterns across CONUS, with decreased volatility in areas of the midwestern US, but notable increases in volatility and volatility-driven risk across many areas of the southeastern and especially western US. This study highlights the risk to many freshwater ecosystems from changes in hydrologic volatility and illustrates how these methods can be used to anticipate changes to non-stationary, changing flow regimes given intensifying hydroclimate, anthropogenic change, and water management.