Why do beavers build dams?

Beavers and their dams play an important role in nature. Because of the dramatic effects their dams have on surrounding ecosystems, these mammals are considered a keystone species. By constructing dams they create wetlands -- lush environments which attract fish, ducks, frogs and other creatures. However, beavers can become a big headache for farmers and landowners

Beaver dams may be good for nature, but why do beavers build them? In short, a dam creates a body of water that makes a relatively safe neighborhood for a beaver family. Since beavers are very good swimmers but fairly slow on land, deeper water creates a habitat where they can find more protection from bears and other predators.

Beavers are nocturnal but they don't spend all night in the ponds or slow-moving streams created by their dams. Instead, they build lodges -- houses where a beaver couple and their children live. Beavers gather sticks, mud, rocks and other available materials to shape these mound-like structures. A group of lodges forms a beaver colony and houses multiple beaver families.
A skilled construction team, a pair of beavers takes just a few days to build a basic dam. A large dam can be up to 10 feet (3.3 meters) high and tens of feet (.33 meters) wide. Just like they make lodges, these rodents use just about anything they can put their paws on to build their dams. This includes sticks, rocks, mud, grass and even trash discarded by humans. To get the large branches for the dam's base a beaver may chew through trees up to several feet (.33 meters) thick.
source : www.howstuffwork.com