Effects of Beaver Dams on Urban Stream Hydraulic Response During Storm Events

Urbanization of landscapes alters watershed hydrology, leading to changes in the natural flow regime of local streams. Runoff from impervious surfaces and routing of stormwater can cause a rapid increase in the volume and velocity of streamflow. This is observed in the hydrograph as a steep rising limb, followed by a high peak, and a rapid falling limb. This rapidly varying streamflow, often referred to as flashiness, can increase erosive forces on the channel bed and banks, leading to channel incision and bank erosion. In areas where the hydrologic regime is less impacted by humans, beaver dams and ponds attenuate storm flow, resulting in a hydrograph that responds more gradually to storms. Currently, however, no studies have been published that examine how beaver dams affect streamflow in urbanized systems. This study seeks to determine whether the presence of beaver dams and ponds in urban stream reaches reduces stream flashiness and decreases the magnitude of storm events. In the Tualatin River Basin, in northwestern Oregon, surface water stage was collected upstream and downstream of two beaver affected stream reaches in the urban portion of the watershed. Continuous stage measurements were combined with channel cross-sectional measurements to derive the wetted cross-sectional area, a timeseries variable that was used as a surrogate for discharge to assess the effects of beaver dams on hydraulic change during storm events. Results showed a moderate decrease in the magnitude of most storm event peaks downstream of beaver-affected reaches, indicating a dampening of streamflow during periods of heavy rainfall. The effects of the beaver dams and ponds on hydraulic variability were quantified using a frequency analysis of the rate-of-change in the wetted cross-sectional area, which tended to reinforce the observed reduction in streamflow variability during storms. The degree of attenuation varied between the two sites, suggesting that other factors influence the efficacy of beaver dams and ponds at dampening streamflow variability during storm events