The animal known for building dams in rivers is the beaver. Beavers are engineering mammals that use branches, mud, and rocks to create dams, which help regulate water flow and create ponds. These ponds serve as habitats for various aquatic species and contribute to a healthy ecosystem by enhancing biodiversity. Beavers have strong incisors that allow them to cut down trees and manipulate their environment effectively. Their dam-building activity is vital for the surrounding ecosystem, as it can lead to increased water retention, improved water quality, and the creation of wetland environments, which benefit many other species.