"The largest North American rodent, the beaver is indeed an active woodcutter and dam builder. Beavers also spend a lot of time in the water, using their tails as rudders and propellers when swimming. The beaver is a perfect example of animal adaptation--in this case to an aquatic environment. Nose and ear valves shut as the animal submerges, an nictitating membranes--similar to those of birds-- serve as natural goggles to protect the eyes from irritation. Meanwhile, the lips close behind its front teeth, enabling it to carry a branch in its mouth without drowning. Beaver can remain submerged as long as 20 minutes. The beaver uses its broad, scaled tail as a rudder when swimming and as a sturdy prop on land." - Minnesota DNR
For more information, see:
http://www.dnr.state.mn.us/mammals/beaver.html