When a dam blocks a river, a river habitat is replaced by a lake habitat. In the Pacific Northwest, large federally owned dams have blocked the migration of coho, chinook, and sockeye salmon from the ocean to their upstream spawning grounds. Some steps are being taken to help the fish around the dams, such as putting them in barges or building fish ladders, but this has only helped a little. Also, when young fish head downriver to the ocean, they can be chewed up in the turbines of the dam. The salmon population in the Northwest currently seems headed for extinction, falling from a population of 16 million to 300,000.
Dams can create large reservoirs over what used to be dry land, producing many problems.
This land is often composed of wetlands, which are important wildlife habitats, and low-lying flood plains, often the most fertile crop land in the area. Population density is typically higher along rivers, leading to mass dislocation of urban centers. The Three Gorges Dam in China is expected to dislocate 1.4 million people. Wildlife habitats destroyed by reservoirs can be especially valuable. Another problem can occur when the area behind the dam is flooded without being properly prepared.