A river system is a number of rivers which consists of one main river, which drains into a lake or into the ocean, with all its tributaries. The source is where the water is coming from, for example, from spring water (waterfalls), mountains etc. The water consistently flows and dumps the water into the ocean after it reaches the end. The water always comes from high to low, emptying into the river, which is called the mouth of the river. Rivers are an important feature of most landscapes, acting as the principal mechanism for the transport of weathered debris away from upland areas and carrying it to lakes and seas, where much of the classic sediment is deposited. River systems can also be deposition, accumulating sediment within channels and on floodplains. The grain size and the sedimentary structures in the river channel deposits are determined by the supply of detritus, the gradient of the river, the total discharge and seasonal variations in flow. Over bank deposition consists mainly of finer-grained sediment, and organic activity on alluvial plains contributes to the formation of soils, which can be recognized in the stratigraphic record as palaeosols. Water flows over the land surface also occur as unconfined sheet floods and debris flows that form alluvial fans at the edges of alluvial plains.
Water flow in rivers and streams is normally confined to channels which are depressions or scours in the land surface that contain the flow. The over bank area or floodplain is the area of land between or beyond the channels that (apart from rain) receives water only when the river is in flood. Together the channel and over bank settings comprise the fluvial environment.