Using beaver dams to restore degraded streams in semi-arid environments

Beaver Castor canadensis were once wide-spread throughout much of the North American continent. Their wide-spread extirpation and the subsequent loss of tens of millions of beaver dams has greatly altered the functioning of streams relative to historical conditions. In many streams, particularly in more arid regions, the wood beaver bring to streams to build dams constitutes a major source of instream wood. Beaver dams alter stream hydrology, geomorphology and sediment transport, together which create conditions that vastly improve opportunities for colonization by certain riparian and instream species. We review the results of our research in eastern Oregon, USA which demonstrate some of the physical and biological changes to stream and riparian habitat resulting from recolonization of degraded streams by beaver. Our observations suggest that beaver dams: 1) change stream, hydraulics so as to create pockets of cool water upwelling downstream of dams, 2) increase sediment retention upstream of dams and aggrade incised streams, 3) raise water tables, increase ground water recharge and increase flow downstream of dams, 4) increase the biomass of fishes, particularly salmonids, and 5) increase the biomass of riparian vegetation. Our results suggest that beaver dams help to restore many important stream ecosystem processes and functions not realized by more traditional instream restoration structures such as large wood and weirs.