Beavers shape the Earth in many ways. First, beavers chew down trees. The limbs from the trees are carried to a body of water. The beavers also carry mud over to the dam. Once the dam is built, it causes a pond of water to form behind the dam. Water which normally flows to other areas is cut off by the dam. Beaver dams are both beneficial and harmful to the environment. They can control floods, create new ecosystems, and remove pesticides from water. However, the dams can block river flow and cause other areas to flood. The loss of trees in the area may cause erosion, too.