A river is a large stream of water that flows in a channel toward a lake, ocean or sea. Usually consisting of freshwater, the water in rivers can come from precipitation, surface runoff, springs, glacial melting, groundwater discharge, or a blend of these processes. While most rivers flow on the surface of the Earth, some flow in caves and caverns; these are known as subterranean rivers. Rivers are a key component in various planetary cycles.
Rivers have been a crucial part of life on Earth since time immemorial. They serve as ecosystems for fish, plants, bacteria and other wildlife. Humans and animals alike use them as sources of drinking water, bathing and food. Humans also use rivers as energy sources in the way of hydropower and as major passageways of transport or navigation. Rivers have even had political influence — many have served as borders of countries and the landforms often act as defensive measures in theaters of war.
Rivers are currently in a state of crisis for several reasons. Building dams and levees help humans but they impair life in the riverine ecosystems and disrupt planetary processes. As a result, many rivers have degraded and some no longer reach the oceans they once did. Moreover, pollution from a variety of sources has diluted (and in some cases, even poisoned) the water that rivers carry. This has harmed the populations found inside rivers and the humans who drink from them. (Photo: Wikimedia Commons)