Changes from source to mouth
A river flows from an upland source. Here the velocity of water is faster than downstream because the river's gradient is steep. Near a river's source the valley has a narrow floor and steep sides (v- shaped).
The middle course of the river has a wider floor and the sides of the valley are more gently sloping. The velocity is slower than the upper stage. However, the channel is wider as the amount of water flowing in it increases as other streams and rivers join it.
The lower course of the river is very gentle sloping, almost flat. The channel is usually at its widest and deepest here because the amount of water flowing within the river is at its greatest.