Habitat and structural factors influencing beaver interference with highway culverts

Abstract

The plugging of highway culverts by beavers (Castor canadensis) creates roadside impoundments that damage and sometimes flood the roadbed. Continually mitigating these problem sites requires considerable time, money, and resources from town, coun- ty, and state highway departments. We initiated this study to develop proactive and long- term approaches to deal with nuisance beavers along roadsides. Our specific objective was to compare culvert and habitat features at plugged and nonplugged culverts. From June to October 1997 and 1998, we sampled 216 roadside sites in New York state: 113 sites where beavers plugged the highway culvert and 1 03 sites where beavers did not plug the culvert but instead constructed an upstream or downstream dam. We used stepwise logistic regression (SLR) to identify key variables associated with plugged culverts. We evaluated classification rates of regression models with measures of sensitivity and speci- ficity. For the combined data set, the logistic function retained culvert inlet opening area (m2) and stream gradient in the final model. Based on these 2 variables, the model cor- rectly classified 790/o of the sites. Our results indicated that installing oversized culverts would have the greatest influence on discouraging beaver plugging activity. Prorated over the service life of culverts, the installation of oversized culverts by highway departments may be more cost-effective than trapping, debris removal, or other short-term options to manage beaver damage to roads.