River

The water in a river is usually restrained to a channel, made up of a stream bed between banks. In larger rivers there is also a wider flood-plain shaped by flood-waters over-topping the feed. Flood plains may be very wide in relative to the size of the river channel. This dissimilarity between river channel and flood-plain can be blurred especially in urban areas where the flood-plain of a river channel can become greatly developed by housing and industry.

The river channel itself typically contains a single stream of water but some rivers flow as several interconnecting streams of water, producing a braided river. All-embracing braided rivers are found in only a few regions worldwide, such as the South Island of New Zealand. They also occur on peneplains and some of the larger river deltas. Anastamosing rivers are similar to braided rivers. They have multiple graceful channels carrying large volumes of sediment. Due to the dynamics of this type of system, they are also quite rare.

A river flowing in its channel is a source of energy which acts on the river channel to change its shape and form. In steep torrential zones this can be seen as erosion channels through hard rocks and the creation of sands and gravels from the devastation of larger rocks. In U shaped glaciated valleys, the consequent 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, loops (meanders) may form through eroding of the river banks and declaration on the inside of bends. Sometimes the river will cut off a loop, shortening the channel and forming an oxbow lake or billabong. Rivers that carry large amounts of deposit may develop conspicuous deltas at their mouths, if conditions permit. Rivers, whose mouths are in brackish tidal waters, may form estuaries. River mouths may also be fjords or rias.