Identification

The beaver (Castor canadensis) is the largest North American rodent. Most adults weigh from 35 to 50 pounds (15.8 to 22.5 kg), with some occasionally reaching 70 to 85 pounds (31.5 to 38.3 kg). Individuals have been known to reach over 100 pounds (45 kg). The beaver is a stocky rodent adapted for aquatic environments. Many of the beaver's features enable it to remain submerged for long periods of time. It has a valvular nose and ears, and lips that close behind the four large incisor teeth. Each of the four feet have five digits, with the hind feet webbed between digits and a split second claw on each hind foot The front feet are small in comparison to the hind feet. The underfur is dense and generally gray in color, whereas the guard hair is long, coarse and ranging in color from yellowish brown to black, with reddish brown the most common coloration. The prominent tail is flattened dorsoventrally, scaled, and almost hairless. It is used as a prop while the beaver is sitting upright and for a rudder when swimming. Beavers also use their tail to warn others of danger by abruptly slapping the surface of the water. The beaver's large front (incisor) teeth, bright orange on the front, grow continuously throughout its life. These incisors are beveled so that they are continuously sharpened as the beaver gnaws and chews while feeding, girdling, and cutting trees. The only way to externally distinguish the sex of a beaver, unless the female is lactating, is to feel for the presence of a baculum (a bone in the penis) in males and its absence in females.

Damage and Damage Identification

The habitat modification by beavers, caused primarily by dam building, is often beneficial to fish, furbearers, reptiles, amphibians, waterfowl, and shorebirds. However, when this modification comes in conflict with human objectives, the impact of damage may far outweigh the benefits.

Most of the damage caused by beavers is a result of dam building, bank burrowing, tree cutting, or flooding. Some southeastern states where beaver damage is extensive have estimated the cost at $3 million to $5 million dollars annually for timber loss; crop losses; roads, dwellings, and flooded property; and other damage. In some states, tracts of bottomland hardwood timber up to several thousand acres (ha) in size may be lost because of beaver. Some unusual cases observed include state highways flooded because of beaver ponds, reservoir darns destroyed by bank den burrows collapsing, and train derailments caused by continued flooding and burrowing. Housing developments have been threatened by beaver dam flooding, and thousands of acres (ha) of cropland and young pine plantations have been flooded by beaver darns (Fig. 6). Road ditches, drain pipes, and culverts have been stopped up so badly that they had to be dynamited out and replaced. Some bridges have been destroyed because of beaver dam-building activity. In addition, beavers threaten human health by contaminating water supplies with Giardia.

Identifying beaver damage generally is not difficult. Signs include dams; dammed-up culverts, bridges, or drain pipes resulting in flooded lands, timber, roads, and crops; cut-d.own or girdled trees and crops; lodges and burrows in ponds, reservoir levees, and darns. In large watersheds, it may be difficult to locate bank dens. However, the limbs, cuttings, and debris around such areas as well as darns along tributaries usually help pinpoint the area.