Wolves, like other animals, are crucial for maintaining a healthy ecosystem but people in almost all of the continental United States conveniently forgot this fact. On the one hand, the wolves disappeared and on the other the populations of other animals exploded. This led to a rapid change in the entire ecosystem of forests.
After a lot of wrangling, it was decided to reintroduce wolves in certain areas. One of such areas was Yellowstone National Park. Wolves were reintroduced after nearly 68 years since the last wild wolf in this park was killed.
This not only brought back the healthy balance in the ecosystem but also changed the rivers. Wolves halted the growth of certain herbivores thereby letting the native flora to re-establish and re-grow, which resulted in increasing the biodiversity by providing food and shelter to a growing variety of plants and animals.