Beaver dam

Dam constructed by beavers

A beaver dam or beaver impoundment is a dam built by beavers; it creates a pond which protects against predators and holds food during winter. They build prolifically at night, carrying mud with their forepaws and timber between their teeth. Dams include other wild materials also found in the region, such as rocks and other organic materials.

From Wikipedia, under CC BY-SA. More on occurri.