Beaver

Yes, Beavers live in streams, dams, wetland, etc.

Beavers are semi-aquatic rodents native to North America and Europe. They are the only living members of the family Castoridae, which contains a single genus, Castor. Genetic research has shown the European and North American beaver populations to be distinct species and that hybridization is unlikely.

Pictures & Information about Beavers:
http://en.wikipedia.org/wiki/Beaver

Beavers are best known for their natural trait of building dams in rivers and streams, and building their homes (known as beaver lodges) in the eventual pond. They are the second-largest rodent in the world (after the capybara).

Beavers continue to grow throughout life. Adult specimens weighing over 25 kg (55 lb) are not uncommon. Females are as large as or larger than males of the same age, which is uncommon among mammals.

Hope this helps =)