The large rodent known for building dams and lodges and being active at night is the beaver.
Beavers belong to the genus Castor, with two living species: the North American beaver and the Eurasian beaver. They use powerful orange incisors to cut trees and branches, then combine wood, mud, rocks, and vegetation to build dams. A dam slows moving water and creates a deep pond that protects the beaver’s lodge and provides underwater access to food.
Beavers are mainly nocturnal and crepuscular, so they are most active after sunset and around dawn. They do not always build dams: in lakes or sufficiently deep rivers, they may live in bank burrows or lodges without constructing an impoundment. Their engineering also creates wetlands that benefit fish, waterfowl, amphibians, and many other species.
A common mix-up is confusing beavers with muskrats or nutria. Those rodents may share wetland habitats, but true beavers are distinguished by their large size, broad scaly tail, tree-felling incisors, dams, and lodges.