Background/Question/Methods: Rivers are drying across the globe as a result of increased human appropriation of freshwater resources and climate change.  Human and climate induced alterations to hydrologic regimes have been indicated in the demise of numerous riverine species.  Here we build on a recent observation that food chain length in rivers increases with basin area and declines with increasing hydrologic variability.  We measure the impacts of dams and ongoing climate change on hydrologic variability in streams across the coterminous US. 

Results/Conclusions: First, using real discharge data from the USGS National Water Information System we quantify the spatial patterns of present day environmental variation across the US.  Using data from the National Inventory of Dams and NOAA we find that both snow depth and dams (reservoir storage) within the sub-basin redden the log-log power spectrum of river hydrographs, or reduce the flashiness of stream hydrographs.  Second, using a novel application of the Variable Infiltration Capacity model (VIC)—a macroscale land surface scheme that predicts runoff in spatially defined river networks—we experimentally alter the presence of dams and snow pack to quantify the independent additive and synergistic effects of hydrologic storage in snow and reservoirs on simulated flow variability.  Finally, we analyze the effects of climate- and dam-related alterations in flow variability on the distribution of piscivorous non-native fishes—again across the entire coterminous US—thereby connecting temporal variation in the environment to a coarse measure of trophic structure at this broad scale.