How do rivers make waterfalls?

High Force Waterfall, River Tees Photo:Wikicommo

 

 

A river that flows over some hard rock followed by some softer rock is likely to make a waterfall. Hard rocks such as granite or basalt are more resistant to erosion; that means they erode very slowly. Softer rocks such as shale or mudstone are less resistant to erosion, this means they erode very quickly. An example of a waterfall in the UK is High Force waterfall on the River Tees; in this example the harder rock is whinstone (dolerite) and the softer rock is carboniferous sandstone.

What are the four types of river erosion?

Erosion means the wearing away of rock. There are four ways in which this can happen:

1. Hydraulic action – the sheer force of the water hitting the rock will weaken and damage the rock. Each cubic metre of water weighs 1 tonne and there are 20 cubic metres of water coming over High Force every second. The crashing water forces air into tiny cracks and causes huge pressure which breaks the cracks wider and forces parts of the rock to become dislodged
2. Attrition – rocks moving around in the water collide and hit each other, this means they smash each other up into smaller pieces
3. Abrasion – rocks being carried in the water scrap against the bed and banks of the river and make wear them down like massive sandpaper rubbing wood.
4. Corrosion – soluble minerals in the rock are dissolved by the passing water, they then move away in the water. Any insoluble minerals might now just fall apart without the soluble ones that were holding them together.

 

A typical diagram to show river erosion. Where would you label each of the four types?

 

As the water flows over the rock below it erodes the softer sandstone away much more quickly. This creates a step. The step becomes steeper and steeper. The river is not just flowing along, it is now plunging down. The hydraulic action of the falling water splashes back and starts to undercut the harder whinstone.

The whinstone overhang is undercut again and again, eventually there is no sandstone underneath the overhang and the whinstone breaks off and falls into the plunge pool. The bits of broken whinstone cause further abrasion to the back wall and scour out the plunge pool. They collide and become smaller through attrition.

The overhang is undercut again and again, eventually falling off into the plunge pool. This happens repeatedly over a long period of time. Every time the overhanging whinstone breaks off the waterfall actually moves backwards (retreats) further upstream (towards the source).

The retreat leaves a very steep-sided narrow valley called a gorge.

Bakota Gorge, Victoria Falls (credit Victoriafalls24.com)