Rivers erode and transport material in several ways. Dissolved ions travel in solution even when the water looks clear.
Corrasion, also called abrasion, occurs when sediment carried by a river scrapes and strikes its bed or banks. It needs abrasive tools as well as moving water. Corrosion is chemical dissolution of susceptible material, such as carbonate rock, by river water. The similar spellings name different mechanisms.
Cavitation occurs when local pressure falls enough for water-vapour bubbles to form and they then collapse as pressure rises. The resulting shocks can damage a boundary. It requires particular high-velocity, low-pressure conditions; it should not be assumed to dominate erosion in ordinary rivers.
Traction rolls or slides larger particles along the bed. Saltation moves smaller bed material in short hops. Suspension keeps fine sediment within the flowing water through turbulence. Solution carries dissolved ions, which do not settle like sand when water slows.
Erosion supplies some of a river's load, while tributaries and slope processes supply more. The size and quantity moved depend on flow energy, turbulence, particle properties and available sediment. During rising discharge, material previously deposited on the bed can be mobilised again.
Rock resistance and jointing affect erosion. Human activity can change discharge and sediment supply: a dam traps load, quarrying exposes loose material, and sand mining removes sediment from the channel. More sediment does not always mean more corrasion; a thick cover can shield bedrock from impacts.
Step by step
Remove material
Separate mechanical corrasion, chemical corrosion and pressure-related cavitation.
Move the load
Identify traction, saltation, suspension or solution.
Match process to evidence
Use grain size, water chemistry, bed conditions and flow, not water colour alone.
Worked example: Clear water still carries material
A limestone stream may look clear while carrying dissolved calcium and bicarbonate. That load is transported in solution. A muddy stream contains suspended particles as well, but the colour does not reveal its entire dissolved load.
Watch out for this
Corrosion and corrasion are two spellings of the same process.
Corrosion dissolves material chemically. Corrasion wears the bed or banks through sediment impacts and scraping.
Check your understanding
Sand grains repeatedly hop along the river bed. Which transport process is visible?
- Solution.
- Suspension.
- Saltation.