A river system is sometimes called a drainage system. It is the whole natural water system in a drainage basin.
Water in a drainage basin usually ends up in the sea, but there are places where the water just evaporates, or flows into an inland lake. Lake Chad, for example, is an example of an endorheic lake, which has no outlet to the sea.
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.
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 floodplain is the land that is covered when the river is in flood. Together the channel and over bank settings comprise the "fluvial environment".