Beavers are the largest rodent in North America, reaching sizes up to 4 feet long and weighing 60 pounds. They are known for their dam building and flat tails. Beaver dams are beneficial to the ecosystem by creating wetlands and habitats for many other animals. The dams also help by creating ponds that purify water by removing sediments and pollutants. Beavers were almost extinct in the early 1900's because of trapping for fur trade.