Habitat and structural factors influencing beaver interference with highway culverts

The plugging of highway culverts by beavers (Castor canadensis) creates roa dside impoundments that damage and sometimes flood the roadbed. Continually mitigating these problem sites requires considerable time, money, and reso urces from town, county, and state highway departments. We initiated this s tudy to develop proactive and long-term approaches to deal with nuisance be avers along roadsides. Our specific objective was to compare culvert and ha bitat features at at plugged and nonplugged culverts. From June to October 1997 and 1998, we sampled 216 roadside sites in New York state: 113 sites w here beavers plugged the highway culvert and 103 sites where beavers did no t plug the culvert but instead constructed an upstream or downstream dam. W e used stepwise logistic regression (SLR) to identify key variables associa ted with plugged culverts. We evaluate classification rates of regression m odel with measures of sensitivity and specificity. For the combined data se t, the logistic function retained culvert inlet opening area (m(2)) and str eam gradient in the final model. Based on these 2 variables, the model corr ectly classified 79% of the sites. Our results indicated that installing ov ersized culverts would have the greatest influence on discouraging beaver p lugging activity. Prorated over the service life of culverts, the installat ion of oversized culverts by highway departments may be more cost-effective than trapping, debris removal, or other short-term options to manage beave r damage to roads.