A river is a natural flowing 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.
Rivers are part of the hydrological cycle. Nile, Amazon, Yangtze, Mississippi, Missouri, Yenisei, Angara, Selenge, Kagera, Volga, Danube, Ural, Dnieper, Don, Pechora, Kama, Amur, Argun, Northern Dvina, Vychegda, Oka, Euphrates, Yukon, Tigris, Belaya, Dniester, Lena, Rhine, Elbe, Donets, Vistula, Tagus, Daugava, Mekong, Loire, 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 (from glaciers). Potamology is the scientific study of rivers while limnology is the study of inland waters in general.