A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In some rare cases a river could flow into the ground and dry up completely at the end of its course, without reaching another body of water.
Small rivers may be called by several other names, including stream, creek, brook, rivulet, and rill. There are no official definitions for generic terms, such as river, as applied to geographic features, although in some countries or communities a stream may be defined by its size.
Many names for small rivers are specific to geographic location; examples are "run" in some parts of the United States, "burn" in Scotland and northeast England, and "beck" in northern England. Sometimes a river is defined as being larger than a creek, but not always: the language is vague.
Rivers are part of the hydrological cycle. Water generally collects in a river from precipitation through a drainage basin from surface runoff and other sources such as groundwater recharge, springs, and the release of stored water in natural ice and snowpacks (e.g. from glaciers). Potamology is the scientific study of rivers while limnology is the study of inland waters in general.
Extraterrestrial rivers have recently been found on Titan. Channels may indicate past rivers on other planets, specifically outflow channels on Mars and are theorised to exist on planets and moons in habitable zones of stars.
Canals and navigations are human-made channels for water, which are generally both referred to in the vernacular as 'canals'. The main difference between them is that a navigation parallels a river and shares its drainage basin, while a canal cuts across a drainage divide.