River erosion and transportation

Overview

How hydraulic action erodes a river channel, competence and the capacity of a river and vertical, lateral and headward erosion.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is abrasion, or corrasion, in a river?

   Model answer

   Abrasion, also called corrasion, is the wearing away of the bed and banks of the channel by the load the river is carrying. Sand, pebbles and boulders are dragged and bounced along the bed and rub against it like sandpaper, scraping material off. It is the most effective erosion process in most rivers and works hardest during high flows when large material can be moved. Where a pebble becomes trapped in a hollow, swirling currents spin it round and drill a cylindrical pothole into the bed.

2. 2

   What is attrition, and why is it different from the other erosion processes?

   Model answer

   Attrition is the collision of the load particles with one another and with the bed, which chips and breaks them so they become progressively smaller, smoother and more rounded. It is different because it wears away the load rather than the channel itself - it does not enlarge the river valley, it only reduces the calibre of the sediment. This is why bedload is angular boulders in the upper course and fine rounded sand and shingle downstream.

3. 3

   Explain how hydraulic action erodes a river channel.

   Model answer

   Hydraulic action is erosion by the sheer force of moving water alone, with no sediment involved. Fast, turbulent water pushes directly against the bed and banks and drags loose material away, and where it strikes a crack in the rock or the bank it forces water and trapped air into it. The trapped air is compressed and then expands explosively as the water pulls back, which repeatedly stresses the rock until it shatters. It is most effective on the outside of meander bends and at the base of waterfalls, and it also undercuts banks made of unconsolidated material so they collapse.

4. 4

   What is solution, or corrosion, as an erosion process?

   Model answer

   Solution, also called corrosion, is the chemical erosion of the channel. River water is naturally slightly acidic because carbon dioxide from the atmosphere and from the soil dissolves in it to form weak carbonic acid, and organic acids from vegetation add to this. This acid reacts with and dissolves soluble minerals in the rock, above all the calcium carbonate in limestone and chalk. It is a chemical reaction, not a mechanical one, so it works continuously even at low flows, and it is far more important on soluble rock than on granite or sandstone.

5. 5

   How does the river carry material in suspension?

   Model answer

   Fine particles of clay and silt are held up within the body of the water by upward eddies in the turbulent flow, which counteract their tendency to settle. They travel at roughly the speed of the water itself rather than along the bed. Suspension usually accounts for the greatest part of the total load by mass in the middle and lower course, and it is what makes a river look brown and opaque during a flood.

6. 6

   What is meant by the solution, or dissolved, load of a river?

   Model answer

   The solution load is the material that the river carries chemically dissolved in the water itself, as ions rather than as particles. It comes mainly from the solution of soluble rocks such as limestone and chalk in the catchment and from dissolved minerals in throughflow and groundwater. It is invisible, it does not settle out when velocity drops, and it is carried right out to the sea at any speed of flow, which is why it can dominate the load of rivers draining limestone country.

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