Why Do Beavers Build Dams?

I was watching Sunrise Earth and saw some beavers in Wyoming building a dam. Why? What makes beavers want to build a dam?
Just to stay busy? I looked it up in the magic box and here's what I found. Based on experiments with both free living and captive beavers the researcher found that the sound of rushing water was as annoying to a beaver as the sound of fingernails on a blackboard is to humans. And that beavers will pile up sticks and mud in any spot they hear that sound until they can no longer hear it. This explains why beavers always pick the narrowest and most shallow section of a stream to build their dams - it's because that's where the noise is. And they continue piling up sticks and mud in that spot until that annoying sound is silenced. In short, beavers build dams because they like peace and quiet.