Book excerpt: Beavers have long been recognized for their ability to increase the ecological function of riparian and aquatic ecosystems. Beaver pond complexes increase geomorphic complexity, surface and groundwater storage, and moderate stream temperature, leading to higher levels of biological and ecosystem diversity. Recently, it has been proposed that beaver may be able to reduce the ecological impacts associated with climate change. In the Pacific Northwest (USA), climate models suggest that temperatures will continue to rise through the next century. Elevated winter temperatures will cause a greater portion of precipitation to fall as rain instead of snow and will lead to earlier snowmelt at higher elevations. With less snowpack, summer low flows are likely to be reduced, potentially threatening aquatic species that rely on cool stream temperatures supplemented by snowmelt. Here, I evaluated whether increasing current beaver populations could reduce these hydrologic impacts of climate change at a variety of spatial and temporal scales. I first developed a predictive beaver habitat model - the beaver intrinsic potential habitat model - as a tool to identify where beaver could exist in a given watershed and to assist in translocation prioritization. Using results from this model, I trapped 91 beaver from lowland areas and relocated them into the Skykomish River watershed, in Washington State, and evaluated how relocated beaver affect stream temperature and surface and groundwater storage. Using these results, I then developed a regional model for western Washington and Oregon that explored the degree to which beaver reintroductions could offset reductions in water availability under various climate scenarios and time frames. The intrinsic potential habitat model identified and ranked potential beaver habitat with a 92 percent accuracy. Population surveys during field validation found beaver to be present in 43 percent of habitable reaches. Through my reintroduction experiment, I found that successful beaver relocations created 243 m3 of surface water storage per 100 m stream reach in the first year following relocation and stored approximately 2.4 times as much groundwater as surface water per relocation reach. On average, stream reaches downstream of newly created beaver dams exhibited a 2.3°C cooling effect in stream temperature during summer base flow conditions. Finally, the regional storage model indicated that despite substantial storage potential from dams, their contribution will likely be small relative to the large amount of snowpack projected to be lost by the end of this century. In snow-dominated basins, beaver may be able to offset small amounts of lost snowpack due to climate change. In basins of the Pacific Northwest that are historically rain dominated, however, beavers have the potential to increase summer water availability by up to 20%. Supporting re-colonization of beavers in areas in which they have not reached carrying capacity could increase hydrologic and thermal resilience to climate change in many basins of the Pacific Northwest.