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. In Brazil, the Tucurui dam was built creating a reservoir in a rain forest region, without the forest first being cleared. Later, as the plants and trees that were submerged began to rot, they reduced the oxygen content of the water, killing off the plants and fish in the water. Moreover, the rotting plants gave off large quantities of methane, a powerful heat-trapping gas. The utility there is now testing underwater power saws to remove the trees.