Beaver Damage Management: Success Evaluation and Risk Assessment Model for Water Level Control Devices

Demands on natural resource managers for non-lethal mitigation of beaver damage (e.g., flooding) are increasing. Water level control devices (WLCD) are designed to reduce damage by facilitating water passage through beaver (Castor canadensis) dams, to preserve valuable wetlands through dam protection and resulting water-level maintenance, and to reduce the need for lethal control. Despite anecdotal information regarding performance, few empirical data on device effectiveness or nonlethality exist. Attempts to assess WLCD performance from multiple jurisdictions were confounded by a lack of standardized evaluation criteria. In addition, standardized WLCD site-choice (risk assessment) criteria are lacking. This study was conducted to: define beaver flowage site characteristics that could explain equivocal performance of WLCDs for maintaining stable water levels or resolving complaints, based on "success" criteria requiring water levels to be held within a predetermined tolerance-range, and develop a predictive risk model to help managers maximize WLCD success, and increase strategy reliability and credibility. Mean WLCD success rates for 15-, 30-, and 45-cm water level fluctuation tolerance thresholds ranged from 74% successful at the 15-cm success threshold (SD =11, range = 52-88) to 98% successful (SD = 2, range = 96-100) at the 45-cm threshold. Beaver activity (i.e. presence, based on fresh mudding, cuttings.and building activities) was the only site variable with a detectable difference relative to water level variation (P = 0.02, 2 df), and AICc model selection revealed that at 30- and 45-cm water level fluctuation tolerance thresholds. Beaver activity negatively influenced WLCD success (w, = 0.99).