The three stages in a river or stream's development are youthful, mature and old. Geologists classify streams along with rivers and view their development as a flowing body of water progressing along the same three stages. Rivers and streams erode and change the landscape around them in their various stages of development and, by doing so, also affect their own course.
A river in its youthful stage follows a relatively straight course. Its gradient is steep and it will usually be found flowing through a V-shaped valley. There may only be a slight or a complete absence of a floodplain, which is the flattened area to the side of the river that is subject to flooding. The flow velocity of a river in its youthful stage is high and there are sometimes rapids along its course.
A mature river does not flow as quickly as a youthful one because it has eroded its river bed down to a lower depth and there is a greater area to accommodate a much larger volume of water to move through it. There are more tributaries feeding a mature river, its floodplains have increased and its channels have eroded to a wider distance. The extent of the channel widening will be greater than the extent of its depth. The Mississippi River and the Thames are examples of mature rivers.
Old rivers flow slowest and their rate of erosion is counteracted by the degree of sediment they deposit. Their course is no longer straight and widened floodplains are a common characteristic. An old river rests in an almost flat valley as a result of the many years of erosion that have taken place. The Nile and the Ganges are examples of old rivers.