Rivers and their tributaries are the veins of the planet, pumping freshwater to wetlands and lakes and out to sea. They flush nutrients through aquatic ecosystems, keeping thousands of species alive, and help sustain fisheries worth billions of dollars.
Rivers are also the lifeblood of human civilizations. They supply water to cities, farms, and factories. Rivers carve shipping routes around the globe, and provide us with food, recreation, and energy. Hydroelectric plants built from bank to bank harness the power of water and convert it to electricity.
But rivers are also often the endpoint for much of our industrial and urban pollution and runoff. When it rains, chemical fertilizer and animal waste peppering residential areas and agricultural lands is swept into local streams, rivers, and other bodies of water. The result: polluted drinking water sources and the decline of aquatic species, in addition to coastal dead zones caused by fertilizer and sewage overload.
Over the course of human history, waterways have been manipulated for irrigation, urban development, navigation, and energy. Dams and levees now alter their flow, interrupting natural fluctuations and the breeding and feeding patterns of fish and other river creatures. Technology and engineering have changed the course of nature, and now we are looking for ways to restore flow and function to the planet's circulatory system.
Read more : http://environment.nationalgeographic.com/environment/freshwater/rivers/