What this lesson teaches
I can explain how rivers erode, transport and deposit material, and what controls those processes.
Syllabus 9173, Topic 2.2, 1. Geomorphic Processes. What influences fluvial processes?
Make a guess
A river in flood slows down as it spreads over its floodplain. Does it drop all of its load at once?
- No, because a slower river carries more.
- Yes, but the dissolved minerals stay.
- No. Big grains drop first; clay travels.
- Yes. A slower river drops its load.
Show the answer
No. Big grains drop first; clay travels.
Different particles drop out at different times. Coarse sand settles as flow slows, while fine clay stays suspended much longer.
A river deposits material when it can no longer carry part of its load, but different particles drop out at different times.
Discharge is the volume of water passing a point each second: channel area times average speed. A steep gradient gives the water energy. Friction from rough beds and banks uses some of that energy. So a steep stretch does not always have the most discharge.
Competence is the largest particle a river can move. Capacity is the total load it can carry. Both usually rise with faster flow. But a fast, clear river may carry little if little sediment is supplied.
Deposition happens when the river can no longer carry part of its load, or when more sediment arrives than it can move. This happens when water spreads out, floods fall, flow meets an obstacle, or a river enters a lake or sea. Deposition is one of the connected processes that change the land surface. Big grains usually drop first.
Fine clay stays suspended easily. But a sticky clay bed can resist erosion. Dissolved minerals only drop out through chemical change. So do not assume the smallest particles are always the easiest to erode.
Plants slow water and trap sediment. Dams trap sediment in reservoirs, so rivers downstream may erode more. Dredging and sand mining also change deposition. These changes can affect the river both upstream and downstream.
- Discharge
- Area times velocity: water volume per second.
- Competence
- The largest particle a river can move.
- Capacity
- The total load a river can carry.
Step by step
Compare flow and supply
Energy available versus sediment arriving.
Find the change
Slower flow or too much supply causes deposition.
Separate grain types
Big grains, clay and dissolved minerals behave differently.
Worked example: A sediment pulse
A landslide sends a pulse of sand into a tributary. In the main river, some sand settles where the channel widens. Fine silt keeps moving downstream. Deposition picks out some particles; it does not stop all transport.
Watch out for this
A river drops all its load whenever it slows down.
Some particles settle, while others stay suspended or dissolved.
Check your understanding
Which statement correctly separates competence and capacity?
- Both mean the speed of the water at bankfull flow.
- Competence is the total load; capacity is the largest particle.
- Competence is the largest particle moved; capacity the total load.
Show the answer
Competence is the largest particle moved; capacity the total load.
Yes. One is about size, the other about amount.