Hydraulic Characteristics and Dynamics of Beaver Dams in a Midwestern U.S. Agricultural Watershed

Populations of North American beaver (castor canadensis) have increased in the past decades throughout the Midwestern U.S., leading to an increase in the frequency of beaver dams in small streams. Beaver dams form ponds and slow water velocity. Multiple dams create a stair-step effect on the water surface profile. The hydraulic and geomorphic influence of beaver dams on streams in the focus of this study. The study area, Little Muddy Creek watershed in eastern Nebraska, is predominantly in agricultural land use. Since the early 1900's, streams in the watershed have undergone deep incision, with new floodplains being established. The main reach of this 3^rd-order watershed was surveyed for beaver dams from 2003 to 2005. Dam locations were documented with mapping grade GPS, integrity of dam structure was noted and upstream and downstream water surface elevations were measured. Failure of dam structure was documented following runoff-producing storms. While some dams were repaired within weeks, others were abandoned and left to degrade, causing a significant and transient change in the water surface profile of the stream. Tests were conducted in the laboratory to determine discharge rating curves for a simulated beaver dam. The upstream and downstream slopes and height of the dam were based on field-surveyed characteristics. Initial tests were run using a smoother surfaced dam, followed by tests with sticks attached to the smooth surface mimicking the roughness of a beaver dam. The rating curve of the beaver dam shifted to the right compared to the smooth dam, indicating that the roughness of the sticks significantly altered the stage-discharge relationship. Field observations showed that the spatial arrangement and hydraulic condition of beaver dams were temporally dynamic in both short and long term scales. Field and laboratory results indicate that beaver dam structures were similar to and could be modeled as broad-crested weirs.