Hydrology Theory- Long Profiles
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.
Hydrology Theory- Cross Profiles
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. As a result there is less turbulence and friction, making the flow of water more efficient.