An Analysis of the Impact of Small Dams on River Fragmentation and Regulation in the Contiguous United States

Abstract

Small dams account for 96% of the more than 50,000 anthropogenic structures on rivers in the United States (US). While dams are critical pieces of infrastructure to human systems, they have contributed to the decline of river connectivity by regulating streamflow dynamics and fragmenting river networks, which threatens the health of freshwater ecosystems. Prior studies of river regulation and fragmentation demonstrated how large dams have impacted river networks. However, small dams account for most structures and 48% of reservoir storage in the US. This study is the first to evaluate the impact of small dams on the river network of the contiguous US (CONUS). We map 51,923 structures onto the US river network to evaluate the current and historical development of fragmentation and regulation. Our results illustrate similar patterns of widespread regulation and fragmentation particularly along major river networks as previous global studies focusing on large dams. The addition of 49,990 small dams in our analysis reveals the full extent of regulation and fragmentation in CONUS. Analysis of small dams highlights the extent to which regulation has expanded into headwater systems over time and their compounding impact on total river regulation. Today, nearly every watershed in the US has high levels of anthropogenic fragmentation and spatial trends have been reversed with the highest fragment densities now occurring in humid regions which can support more structures. In these locations with the largest change in fragment density, small dams account for over 70% of the change.