The broad, paddlelike tail is flattened horizontally, covered with large scales, and mostly naked. The anal and urogenital orifices open into a common cloaca. The anal glands are located posterior to the castors and open independently into the cloaca. Both pairs of glands release odoriferous products that are deposited on piles of stones or mud. In males a baculum is present.
Females have four pectoral mammae.
The incisors are strongly developed, and the high-crowned cheek teeth have flat grinding surfaces and numerous enamel folds, but they do not grow throughout life. The skull is massive. Beavers are semiaquatic. A dam provides an area of still, deep water where a lodge can be conveniently constructed and protected from terrestrial predators and where building materials and food supplies can be easily floated and kept from being washed away. Dams might not be constructed by beavers living near lakes and rivers that originally were sufficiently quiet and deep.