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 looks completely clear, yet it is carrying limestone downstream. How?
- By rolling it along the bed.
- As dissolved minerals in the water.
- It cannot; clear water carries nothing.
- By bouncing grains in short hops.
Show the answer
As dissolved minerals in the water.
Solution carries dissolved minerals that you cannot see. They travel even in clear water and do not settle out when the river slows.
Rivers erode and carry material in several ways, and dissolved minerals travel in the water even when it looks clear.
Fluvial processes are the connected ways rivers erode, transport and deposit material. Corrasion, or abrasion, is when stones carried by the river scrape and hit its bed and banks. It needs sediment as tools. Corrosion is river water chemically dissolving rock, such as limestone. The two names look alike but are different.
Cavitation is erosion by collapsing air bubbles. In very fast water, pressure drops and bubbles form. When they collapse, the shock can damage the bed. It needs special high-speed conditions, so it is rare in ordinary rivers.
Rivers transport their load in four ways. Traction rolls or slides large stones along the bed. Saltation bounces smaller particles in short hops. Suspension carries fine sediment in the water. Solution carries dissolved minerals, which do not settle out when the water slows.
Load also comes from tributaries and slopes. How much moves depends on flow energy, particle size and supply.
Rock hardness and cracks affect erosion. People change rivers too. Dams trap sediment, quarries add loose material, and sand mining removes it. More sediment does not always mean more corrasion: a thick layer of sediment can shield the bedrock.
- Corrasion vs corrosion
- Scraping by stones vs chemical dissolving.
- Four transport ways
- Traction, saltation, suspension, solution.
- Cavitation
- Collapsing bubbles in very fast water; rare.
Step by step
Remove material
Corrasion, corrosion or cavitation?
Move the load
Traction, saltation, suspension or solution?
Match to evidence
Grain size, water chemistry and flow, not colour.
Worked example: Clear water still carries material
A limestone stream may look clear but carry dissolved calcium. That load moves in solution. A muddy stream also carries suspended particles, but its colour does not show the dissolved load.
Watch out for this
Corrosion and corrasion are two spellings of the same process.
Corrosion dissolves rock chemically. Corrasion wears it away with stones.
Check your understanding
Sand grains keep hopping along the river bed. Which transport process is this?
- Saltation.
- Solution.
- Suspension.
Show the answer
Saltation.
Yes. Grains move in short hops.