Cross-profiles of a river
What is a cross-profile?
River cross profiles show you a cross-section, taken sideways, of a river’s channel and/or valley at certain points in the river’s course.
A channel cross-profile only includes the river whereas a valley cross-profile includes the channel, the valley floor and the sides of the valley.
How and why does a channel cross-profile change downstream?
How does the cross-profile of a river change?
There are three main sections along the long profile of a river. These are the upper, middle and lower course. Each has its own channel cross-profile features.
The upper course of the river
The middle course of a river
The lower course of the river
Why does it change?
In the upper course, the river erodes its bed by hydraulic action and abrasion. As the river flows downstream it is joined by tributaries, increasing the volume of water, velocity and therefore its erosive power. This enables it to cut a deeper channel as it flows downstream.
Downstream, the channel becomes wider as the gradient becomes more gentle leading to less vertical erosion. By the middle course of the river lateral erosion becomes the dominant type of erosion. The channel becomes wide because of lateral erosion.
How and why does a valley cross-profile change downstream?
How does it change?
In the upper course, the valley cross-profile is narrow and steep-sided. The river takes up most of the valley floor. In the middle course, the valley becomes wider due to lateral erosion. By the lower course, the valley is almost flat, consisting of a wide flood plain.
Why does it change?
A steep, V-shaped cross-profile is typical in the upper course. This is because of vertical erosion by the river combined with weathering and mass movement of the valley slopes.
In the middle course, the river is flowing through lower lying land. The gradient is gentler, so the river begins to meander (bend). As it does this the dominant type of erosion becomes lateral, eroding the valley sides. The makes the valley broader. Also, the rate of weathering increases on the softer rocks of the valley sides.
In the lower course, the river is passing through the low-lying country. Flooding results in deposition building up the flood plain and this, along with migrating meanders builds up and widens the valley further.