Beavers are ecosystem engineers that modify watersheds and river flow regimes. By building dams, they restore channel-floodplain connectivity, stabilizing flows and increasing groundwater storage. Notably, beavers may be considered a Nature-based Solution to water scarcity in the dry U.S. West, where snow droughts are reducing snowpack and advancing snowmelt in free-flowing mountain streams - often inducing hydrologic regime shifts from perennial to intermittent hydrographs. In California, the Department of Fish and Wildlife is working to restore beavers to Sierra Nevada watersheds where they could help capture and store snowmelt water to sustain flows during drought. However, despite promising ecological and hydrological implications, the consequences of this restoration on California’s watersheds and freshwater resources remain largely unknown. This work seeks to address this knowledge gap by adapting the Soil and Water Assessment Tool Plus (SWAT+), a process-based hydrologic model, to simulate the effects of beaver dams on water storage and river flows. The modeling approach is developed and evaluated in two hydrologically similar headwater catchments in the Lake Tahoe Basin. Ward Creek and Blackwood Creek watersheds are adjacent, have very similar drainage areas (25 and 29 km2, respectively), as well as aspect, soil, land use, and climate. Additionally, each catchment has a USGS in-situ stream gauge measuring daily discharge where the creeks drain to Lake Tahoe. No reservoirs are present in either catchment, and beaver dams are only observed in Blackwood Creek throughout the duration of the streamflow records. Despite these watersheds being physical “twins”, they experience different flow regimes, with Ward Creek becoming increasingly intermittent over time and Blackwood Creek maintaining perennial flows. This study deploys a hydrologic model to assess whether this difference in flow regime can be attributed to beaver presence on the landscape. With the potential for transferability across the Sierra Nevada, this tool can help inform restoration planning by identifying where beavers are most likely to counteract the ongoing and projected effects of climate change by preserving perennial flow in free-flowing headwater streams.