A river changes shape as it flows from its source (where a river starts) to its mouth (where a river flows into a sea or lake). The shape of both the long profile (a slice through the river from source to mouth) and the cross profile (a slice across the river) changes.
Long profile of a river
The source of a river is often – but not always – in an upland area. Near the source, a river flows over steep slopes with an uneven surface. It often flows over a series of waterfalls and rapids. Highland areas are usually composed of hard igneous rocks, which are ideal for forming such features.
As a river flows down steep slopes the water performs vertical erosion. This form of erosion cuts down towards the river bed and carves out steep-sided V-shaped valleys.
As the river flows towards the mouth, the slopes become less steep. Eventually the river will flow over flat land as it approaches the sea.
The discharge (amount of water flowing) will increase as the river approaches the sea.
Cross profiles of a river
Near the source of a river there is more vertical erosion as the river flows downhill, using its energy to overcome friction (A). As a result the channels are narrow and shallow and may contain large boulders and angular fragments eroded and weathered from the steep valley sides. The sediment in the river creates turbulence and friction.
As the river approaches the mouth, velocity and energy increase due to increased discharge. The river performs more lateral erosion making the channel wider, and smoother (B) and (C). As a result there is less turbulence and friction, making the flow of water more efficient.