Hydrologic and temperature effects of reintroduced beaver in headwater streams

Abstract:

Beaver are well known for their ability to create pond and wetland complexes that provide many benefits for aquatic species. Their dam-building increases riparian complexity, habitat availability, biodiversity, and ecosystem resilience. Despite previous studies that have evaluated and quantified these beaver benefits, there remains uncertainty and active debate around how beaver affect processes such as water storage and stream temperature. Additionally, the reintroduction of beaver into vacant streams as a restoration tool with the intent of moderating stream temperature and storage has not been previously assessed quantitatively. Prior to widespread trapping and eradication in the early 1900’s, beaver were once ubiquitous throughout much of Washington State. Although populations are recovering in many areas, Washington’s Cascade Mountains continue to have vacancy in many suitable streams. This continued vacancy represents a future opportunity to increase the amount of riverine and emergent wetlands, habitat diversity, ecosystem function, and hydrologic stability. The objectives of our study were to quantify the surface and subsurface storage potential of beaver complexes during summer low-flow conditions, identify how beaver complexes affect stream temperature, and evaluate the restoration potential of beaver reintroduction as a tool to increase ecosystem resilience to climate impacts. Between 2014 and 2016, we trapped 91 beaver in Washington’s Puget Sound lowlands and relocated colonies into headwater stream reaches of the Cascade Mountains. We evaluated change in surface and groundwater storage as well as stream temperature at pre- and post-release sites. We also evaluated beaver damming effects on stream temperature at sites with a variety of beaver structures and conditions present. Successful relocations created 243 m³ of surface water storage per 100 m stream reach in the first year following relocation. Dams raised water table elevations by up to 0.33 m and stored approximately 2.4 times as much groundwater as surface water per relocation reach. Stream reaches downstream of dams exhibited, on average, a 2.3˚C decrease in stream temperature during summer base flow conditions. We also assessed how dam age, condition, and pond morphology influenced stream temperature at wetland complexes constructed by naturally-colonized beavers. Small, abandoned dams produced more warming than all other dam morphologies, including those created by relocated beavers. Our findings demonstrate that dam-building can increase water storage and reduce stream temperatures in the first year following successful beaver relocation. When deciding where to relocate beavers, however, initial morphology of candidate reaches may determine the type and magnitude of response. Relocation to reaches with existing small, abandoned ponds may address cooling goals by converting them from warming to cooling reaches, whereas relocation within large, abandoned complexes or vacant habitat may result in greater water storage. Thus, if the ecologically limiting factors in candidate reaches are known, such as high temperatures or low in-stream flow, beaver reintroduction may be targeted towards specific restoration objectives.