River processes Lesson objectives To know the river

River processes Lesson objectives: • To know the river processes of erosion, transportation and deposition • To understand where on the rivers course these processes happen © www. teachitgeography. co. uk 2017 29004 1

Types of erosion • There are four main types of river erosion. • On your diagram add descriptions to ensure you understand the differences. Hydraulic impact in cracks and joints Attrition Solution Abrasion © www. teachitgeography. co. uk 2017 29004 2

The four main forms of river erosion • Hydraulic action - the sheer force of the moving water against the banks can cause air to be trapped in cracks and crevices. The pressure weakens the banks and gradually wears it away. • Abrasion - rocks carried along by the river scrape, scour and rub along the bed and banks and wear down the river like sandpaper. • Attrition - rocks being carried by the river smash together and break into smaller, smoother and rounder particles. • Solution – minerals in the rocks are dissolved by weak acids in the river water. Particularly in rocks like chalk and limestone, soluble particles are dissolved into the river water. © www. teachitgeography. co. uk 2017 29004 3

Types of transportation • There are four main types of river transportation. • On your diagram add descriptions to ensure you understand the differences. Suspension Solution Traction Saltation River bed © www. teachitgeography. co. uk 2017 29004 4

The four main river transport processes • Traction - large stones and boulders are rolled along the river bed by water moving downstream. Mainly when there is high discharge and therefore high energy levels. • Saltation - small pebbles and stones are bounced along the river bed. As these particles land they dislodge other particles causing further saltation. • Suspension - fine light materials, sand silt, are carried along in the flow of the water. This gives rivers their brown appearance. • Solution - minerals are dissolved in the water and carried along in solution. © www. teachitgeography. co. uk 2017 29004 5

Rivers need energy to transport! Rivers need energy to transport material, and levels of energy change as the river moves from source to mouth. • When energy levels are very high, large rocks and boulders can be transported. • When the channel is steep • During times of flood • When energy levels are very low, only small particles can be transported (if any). • When the velocity drops as rivers enter a lake or the sea • When the channel is shallower and there is more friction © www. teachitgeography. co. uk 2017 29004 6

Deposition Rivers deposit when they do not have the energy to carry material anymore, this happens when: 1. The gradient of the river reduces, e. g. when the river meets the sea or a lake. 2. The river discharge is less and there is not as much water to carry its load, e. g. during a dry period of weather. 3. The river channel is shallower and therefore there is more friction, e. g. on the inside of a meander bend. 4. The river floods and overtops its banks, meaning the velocity of the water decreases. © www. teachitgeography. co. uk 2017 29004 7

Usually the largest material is deposited first and then smaller particles are deposited as they move downstream. © www. teachitgeography. co. uk 2017 29004 8