A river erodes its channel, transports a load and deposits it again, and the balance between the three changes along its length. In IGCSE Geography you are asked to describe that change and explain it, usually with a table, a photo or a short description of a reach.
This lesson sits in rivers and drainage. It comes first because the later lessons on cross-sections, landforms and hydrographs all rely on the same idea: energy decides what a river does.
What are the processes?
Erosion is the wearing away of the bed and banks. There are four processes:
- Hydraulic action: the force of water pushes into cracks, compresses air and loosens rock.
- Abrasion: the load scrapes and rubs the bed and banks.
- Attrition: stones in the load collide and become smaller and rounder.
- Solution: soluble rock, such as limestone, dissolves in the water.
Transport moves the load downstream. Heavy stones roll along the bed (traction). Smaller pebbles and sand bounce along in short hops (saltation). Fine silt and clay are carried within the water (suspension), and dissolved minerals travel in solution.
Deposition happens when the river no longer has the energy to carry its load. That is usually because velocity or volume has dropped.
How do you explain a change along a reach?
Use the same three-link chain every time: energy, process, evidence.
- State what changes: gradient, velocity, channel size or load.
- Say what that does to the river’s energy.
- Name the process that follows, and point to the evidence in the data.
Worked example
All figures below are invented for practice. A river has three sample sites. The table shows what a student recorded.
| Site | Distance from source | Mean stone length | Stone shape | Velocity |
|---|---|---|---|---|
| A | 2 km | 24 cm | Angular | 0.9 m/s |
| B | 30 km | 9 cm | Sub-angular | 0.7 m/s |
| C | 70 km | 2 cm | Rounded | 0.4 m/s |
Question: Describe and explain the change in the stones from A to C.
Describe: The stones become much smaller, from 24 cm to 2 cm. That is a fall of 22 cm. They also change from angular to rounded.
Explain, link 1 (cause): Stones are carried downstream and collide with each other and the bed. Attrition knocks off corners and breaks stones into smaller pieces.
Explain, link 2 (energy): Velocity falls from 0.9 to 0.4 m/s. With less energy, the river cannot carry the largest stones, so larger ones are deposited first and only finer material continues.
Evidence: Rounded stones at C show a long journey, while angular stones at A suggest a short one, close to the source.
Check of the sums: 24 − 2 = 22 cm, and 0.9 − 0.4 = 0.5 m/s. Both are right.
The mistake to watch for
A common answer is: “The stones get smaller because of abrasion.”
Abrasion wears the channel. Attrition wears the stones. The student has named a real process but applied it to the wrong thing, and an examiner will not credit it. The corrected sentence is: “Attrition makes the stones smaller and rounder as they collide while being transported.”
A second slip is to say deposition happens “because the river erodes less”. Deposition happens because energy has fallen, and the load becomes too heavy for the river to carry.
Check yourself
All data are invented.
1. Name the process: a river flowing over limestone gradually dissolves the rock.
Show answer
Solution. Limestone is soluble, so the water removes it chemically.
2. Which transport process moves sand grains in a series of short bounces along the bed?
Show answer
Saltation. Traction is rolling, and suspension is carried within the water.
3. After a storm, a river slows as it enters a wide, flat valley, and gravel is left on the bed. Explain why in two linked sentences.
Show answer
The river slows, so it loses energy and can no longer carry the heavier gravel. The gravel is therefore deposited on the bed, with the largest pieces dropping first.
Where this leads next
Next, interpret a channel cross-section to see how channel shape controls velocity and discharge. Test the whole module in the rivers and drainage practice set. The statistics and distribution explorer is useful for averaging stone measurements.
Students often name the right process but attach it to the wrong thing, as abrasion and attrition show. A teacher in online one-to-one Geography tuition can catch this in your own answers.