A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including stream, creek, brook, rivulet, run, tributary 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; one example is "burn" in Scotland and northeast England. Sometimes a river is said to be larger than a creek, but this is not always the case, because of vagueness in the language. Rivers are part of the hydrological cycle. Water within a river is generally collected 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 snow packs (e.g., from glaciers). Potamology is the scientific study of rivers while limnology is the study of inland waters in general. No extraterrestrial rivers are currently known, though large flows of hydrocarbons described as rivers have recently been found on Titan. Channels may indicate the presence of past rivers on other planets, specifically outflow channels on Mars and are theorised to exist on planets and moons in habitable zones of stars.
A river begins at a source and ends at a mouth, following a path called a course. The water in a river is usually confined to a channel, made up of a stream bed between banks. In larger rivers there is also a wider floodplain shaped by flood-waters over-topping the channel. Floodplains may be very wide in relation to the size of the river channel. This distinction between river channel and floodplain can be blurred especially in urban areas where the floodplain of a river channel can become greatly developed by housing and industry. Rivers can flow down mountains, through valleys (depressions) or along plains, and can create canyons or gorges in some cases. The term upriver (or upstream) refers to the direction leading to the source of the river, which is against the direction of flow. Likewise, the term downriver (or downstream) describes the direction towards the mouth of the river, in which the current flows. The river channel typically contains a single stream of water, but some rivers flow as several interconnecting streams of water, producing a braided river. Extensive braided rivers are now found in only a few regions worldwide, such as the South Island of New Zealand. They also occur on pen plains and some of the larger river deltas. Anastamosing Rivers are similar to braided rivers and are also quite rare. They have multiple sinuous channels carrying large volumes of sediment. There are only two cases in the world of rivers dividing and the resultant flows ending in different seas, one being the Bifurcation of Nerodime river in Kosovo.
The River Cam from the Green Dragon Bridge, Cambridge UK
A river flowing in its channel is a source of energy which acts on the river channel to change its shape and form. In 1757, the German hydrologist Albert Brahms empirically observed that the submerged weight of objects that may be carried away by a river is proportional to the sixth power of the river flow speed. (This formulation is also sometimes called Airy's law.) thus, if the speeds of flow were doubled, the flow would dislodge objects with 64 times as much submerged weight. In mountainous torrential zones this can be seen as erosion channels through hard rocks and the creation of sands and gravels from the destruction of larger rocks. In U-shaped glaciated valleys, the subsequent river valley can often easily be identified by the V-shaped channel that it has carved. In the middle reaches where the river may flow over flatter land, meanders...
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