COS 246-6 - Beavers in the Sierra Nevada: modeling the potential of natural dam builders to enhance drought resilience in California, United States

Abstract: In recent years, stream conservation practitioners across the western United States have increasingly focused on restoring populations of beavers (Castor canadensis). Beavers, once ubiquitous across North America, are ecosystem engineers that alter riparian and aquatic ecosystems by building dams, which can help restore incised streams, store carbon, create diverse habitat, raise water tables, and increase fire resilience. In California, where drought is a persistent issue and beaver populations are relatively low, restoring beaver populations could offer an ecologically beneficial way to increase groundwater recharge and water storage at a watershed scale. The goal of our study is to assess how restoring beaver populations in the Sierra Nevada mountains could influence water resources in the state of California. To address this goal, we used the Beaver Restoration Assessment Tool (BRAT) to model historical and current beaver dam building potential in watersheds across the Sierra Nevada. This model estimates the potential density of beaver dams in a stream network using a combination of high and low flow measurements, channel gradient, vegetation type and proximity, and stream size. We then used BRAT model outputs (dams per kilometer) as inputs in NASA's Land Information System (LIS) framework, which is a tool that allows us to run an hydrological model forced by atmospheric forcings and combined with terrestrial hydrology datasets. LIS helps us assess the movement of water through a watershed over time, which allows us to quantify how increased beaver dam density could change shallow groundwater storage and water retention time at a landscape scale. Initial BRAT model results from the Upper Merced watershed in central California show that beaver dam building capacity in the watershed has decreased from historical capacities of 10.7 dams/km to current potential capacities of 4.7 dams/km, a more than 56 percent decline in dam building capacity. We expect to see similar declines across the state. Despite declines in capacity from historical levels, we hypothesize that if beaver dam building activity in California reached its full current capacity, shallow groundwater storage and water retention time would increase significantly. Our modeling approach is the first to our knowledge combining beaver dam building capacity models with advanced terrestrial hydrology models, thus allowing us to quantify the benefits of this ecosystem engineer at scale. This study will offer important context on how restoring beaver populations in California could impact water resources throughout the state and the western US.