Comparing Potential Beaver-related Sediment Storage to Human-built Reservoir Sediment Storage in the Rocky Mountain Region, USA

Beavers modify rivers and floodplains by building dams, increasing sediment and carbon storage, and creating habitat for biota. In the Rocky Mountain region, USA, beavers are being reintroduced for river restoration, with goals that include promoting sediment storage in incising reaches and decreasing downstream fluxes of sediment and associated constituents. Human-built dams that impound rivers and create reservoirs also store sediment within river corridors, but they lack the significant ecosystem benefits associated with beaver activity. Here, we ask: what is the relative magnitude of potential beaver-induced sediment storage compared to sediment storage in human-built reservoirs? We modeled beaver dam capacities in river corridors with the beaver restoration assessment tool (BRAT), which leverages landcover and topographic geospatial data to determine the maximum potential density of beaver dams on the river network. Based on potential dam density, we estimated potential beaver-related sediment storage across the region using field-derived statistical relationships between pond characteristics and sediment volumes. We used national sedimentation models trained on repeat reservoir survey data to estimate reservoir sediment storage. These sedimentation models are built upon ResNet, a dammed reservoir network that links US dams with the US National Hydrography Dataset. We found that if beaver were at 100% capacity, the magnitude of potential beaver-related sediment storage in the Rocky Mountain region would be ~2.8 km³. This is similar to sediment storage in human-built, dammed reservoirs in the region modeled for the years 2025 (~2.2 km³) and 2050 (~2.8 km³). For some drainage basins in the region, potential beaver-related sediment storage exceeds sediment storage in human-built reservoirs. To date, most studies of beaver-induced sediment storage in river corridors have occurred over relatively small spatial extents (individual ponds, reaches, and small watersheds). However, estimating potential sediment storage due to beaver over larger spatial extents is important for informing management of landscape-scale sediment storage, especially if a goal is to reduce downstream sediment loading to reservoirs. Our results allow for assessing the relative impact of beaver dams versus human-built dams on distributed sediment storage at the watershed and regional scales.