A river is a large natural waterway. It is a specific term in the vernacular for large streams, stream being the umbrella term used in the scientific community for all flowing natural waterways. In the vernacular, stream may be used to refer to smaller streams, as may creek, run, fork, etc.
Passage via a river or stream is the usual way rainfall on land finds its way to the ocean or other large body of water such as a lake. A river consists of several basic parts, originating from headwaters or a spring at the source, that flow into the main stream. Smaller side streams that join the river are tributaries. Water flow is normally confined to a channel, with a bottom or bed between banks. The lower end of a river is its mouth.
A river conducts water by constantly flowing perpendicular to the elevation curve of its bed, thereby converting the positional energy of the water into kinetic energy. Where a river flows over relatively flat areas, the river will meander: start to form loops and snake through the plain by eroding the river banks. Loops that are formed are sometimes cut off, forming a shorter river channel and leaving a remnant, oxbow lake. Rivers that carry large amounts of sediment develop conspicuous deltas at their mouths. Rivers whose mouths are in saline tidal waters may form estuaries.
Where a river descends quickly over sloped topography, rapids with whitewater or even waterfalls occur. Rapids are often used for recreational purposes (see Whitewater kayaking). Waterfalls are sometimes used as sources of energy, via watermills and hydroelectric plants.
Rivers begin at their source in higher ground, either rising from a spring, forming from glacial meltwater, flowing from a body of water such as a lake, or simply from damp, boggy places where the soil is waterlogged. They end at their base level where they flow into a larger body of water, the sea, a lake, or as a tributary to another (usually larger) river. In arid areas rivers sometimes end by losing water to evaporation and percolation into dry, porous material such as sand, soil, or pervious rock.
The area drained by a river and its tributaries is called its watershed.