People have constructed many large dams for the purpose of flood control, water supply, and electricity generation. However, dams can have negative environmental impacts by changing the environment, affecting natural habitats in the dam areas. For example, the Elwha River in the Pacific Northwest of the United States lost nearly all of its salmon due to the construction of the two dams. A 108-foot dam and a 210-foot dam blocked their path to the freshwater stream of their origin to lay their eggs.  The dams were removed in 2014 to restore the destroyed ecosystem, and salmon began to make a return to the river after the removal.