W-2

The **long profile** shows how the river changes in height along its course.
 * __Long and changing cross profiles__**

The diagram below shows a theoretical long profile and cross profile from the source to the mouth.

The steep reduction in height near the source gives way to a more gradual reduction further downstream, giving a typical concave profile. The river has much potential energy near the source due to the steep drop. Later on, this is replaced by energy from a large volume of water. However, such a perfect long profile is rare. This is due to land being uplifted, sea-level changing and bands of hard and soft rock crossing the path of the river.

As the river flows downstream, its valley changes shape and the **cross profile** from one side of the valley to the other clearly shows this. Generally the cross profile shows the valley becoming wider and flatter, with lower valley sides.

You can split the long profile into three parts: upper, middle and lower. They have their own distinctive characteristics as shown below in the diagram.
 * __Key terms__**
 * Long profile**: a line representing the course of the river from it source (relatively high up) to its mouth where it ends, usually in a lake or the sea, and the changes in height along its course.


 * Cross profile**: a line that represents what it would be like to walk from one side of a valley, acorss the channel and up the other side