Wolves reintroduced to Yellowstone changed the shape of rivers

When wolves were reintroduced to Yellowstone National Park in 1995, after a 70-year absence, their presence changed not just elk populations but the entire ecosystem — a phenomenon called a 'trophic cascade'. Elk stopped grazing in valley bottoms and riverbanks where wolves could chase them. Vegetation regrew along riverbeds, roots stabilised the banks, and rivers stopped meandering, cutting deeper channels. Beavers returned, building dams that created wetland habitats for otters, ducks, and fish. The wolves, by changing animal behaviour as much as numbers, literally changed the geography of the park.

Source: Ripple & Beschta, Biological Conservation (2012)