Things are improving in the United States, where environmental law, public pressure, and technological advances have combined to restore some of the natural order of parian ecosystems. But developing countries cannot afford such expensive solutions. And in drought-ridden countries, reservoirs, although a source of environmental problems and human health risks, are vital for community and urban water storage, and even more are needed to respond to population expansion and the push for agricultural development. Reservoirs can trap nutrient-laden sediments behind the dam, reducing reservoir volumes and accelerating a cycle of eutrophication. Tropical reservoirs and irrigation networks are also ideal breeding grounds for mosquitoes, snails, and flies, the vectors that carry malaria, schistosomiasis, and river blindness.
Dams are an important source of electrical power in many countries.
Hydropower is efficient and reliable, and, once constructed, hydro plants are inexpensive to run. But questions have arisen about the environmental impact of hydropower. While hydropower produces little carbon dioxide, large, shallow reservoirs, especially in the tropics, can generate large amounts of greenhouse gases from the decay of biomass. Dams can protect crop fields from flooding and, in some regions, may supply enough irrigation for a year-round growing season. But irrigation in arid climates often results in salinization of the soil and water, stunting or killing crops unless even more water is used to flush the salt out.