Beavers have a tremendous impact on ecosystems. Dams alter the flow of rivers and can flood hundreds of acres. Dams prevent erosion and raise the water table, which helps purify the water as silt builds up and breaks down toxins, according to ADW. As sediment and debris build up, carbon increases and nitrogen decreases. The chemical changes alter the type of invertebrates, and the new water source attracts new species of birds, fish and amphibians. Flooded timber dies off and a forest becomes an open water ecosystem. Over time, abandoned dams decay, and meadows appear.