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.