The Geomorphic and Hydrologic Impact of Beaver Dams on Headwater Streams in Northeastern Connecticut and Implications for River Restoration

The billion-dollar river restoration industry relies primarily on the concept of a free-flowing river to set restoration targets. However, rivers include natural barriers such as beaver dams, wood jams, glacial deposits, and bedrock constrictions. Following European colonization, most of these barriers were removed; some were replaced with far more homogenous ones such as human dams and road crossings. Although the biota intended to benefit from restoration evolved in rivers with natural barriers in place, little is known about the functions of the barriers that have been lost. Beaver dams—the subject of this dissertation—are just one type of the many natural barriers that should be considered by river restoration efforts. Chapter 2 presents a conceptual model of a river network that includes barriers, generating the fundamental hypothesis that intact and failed barriers create patchy features that store and release water and sediment. In chapter 3, a detailed geomorphic comparison of free-flowing and impounded channels shows that beaver dams decouple stream reaches, where distinct differences in adjacent channel reaches are explained by the presence of beaver dams. Observations of fine sediment deposits in steep reaches downstream of dams and of net sediment losses from old ponds support the hypothesis that beaver ponds store and release sediment. The hydrologic study of chapter 4 shows that the river channel through a beaver meadow loses water during rain events and subsequently gains water during recession, confirming the hypothesis of storage and release of water. Additional water gains during storm recessions in excess of the volume lost during the events, along with significantly lower runoff rates in the meadow channel during the events, suggest additional storage and subsequent release of upland runoff by the meadow. Chapter 5 examines summer water temperature at the streambed, which further demonstrates the patchiness generated by the intact and failed beaver dams. A distinct cold pool exists at a scour hole generated by a dam failure, and beaver dams buffer water temperatures upstream. As chapter 6 concludes, this patchiness should be further researched as a target for river restoration efforts where natural dams cannot be directly restored