River processes — erosion, transportation and deposition

Rivers shape the landscape through three linked processes: erosion (wearing away material), transportation (moving material) and deposition (dropping material). CCEA examiners expect you to name each mechanism precisely and to explain how the balance between these processes changes along the course of a river.

Erosion — four mechanisms

Hydraulic action: the sheer force of moving water against the river bank and bed. It traps and compresses air in cracks, which eventually splits the rock. Most effective in fast-flowing water (upper course, waterfalls).

Abrasion (corrasion): the river uses its load of stones and sediment as sandpaper to scrape and grind the bed and banks. Responsible for most of the vertical and lateral cutting of river valleys.

Attrition: particles carried by the river collide with each other, gradually becoming smaller and more rounded. This is why sediment near the mouth is fine and smooth compared to the angular boulders of the upper course.

Solution (corrosion): minerals in rocks dissolve in the river water. Particularly effective on limestone landscapes (chalk, carboniferous limestone). The dissolved material is carried invisibly in solution.

Remember the four: H-A-A-S — Hydraulic action, Abrasion, Attrition, Solution.

Transportation — four processes

Traction: the largest boulders and pebbles are rolled along the river bed by the current. Requires high energy (steep gradient, flood conditions).

Saltation: smaller pebbles and coarse sand bounce along the bed in a series of hops. Characteristic of medium-energy sections.

Suspension: fine silt and clay particles are carried within the body of the water, making rivers look brown or muddy after rainfall. Most common form of transport by volume.

Solution: dissolved minerals are transported invisibly. Not to be confused with the erosion process of the same name — here the minerals are already in solution and being moved.

Remember: T-S-S-S or "The Salmon Swims Sideways."

Deposition

Deposition occurs when a river loses energy and can no longer carry its load. Energy loss happens when:

• The gradient (slope) decreases (e.g. leaving the mountains for the floodplain).
• Discharge decreases (e.g. during drought, or where a tributary has branched away).
• The river enters a sea or lake and momentum is lost.
• The river meets an obstacle (e.g. a bridge pier).

Hjulstrom curve: this graph shows the relationship between velocity and the particle sizes that can be eroded, transported, or deposited. Key points: (1) Fine clay particles require surprisingly high velocities to erode because they are cohesive — they stick together. (2) Sand is easiest to erode. (3) Once particles are in transport, they can be carried at velocities lower than those needed to pick them up.

Changing processes along the river's course

Upper course: steep gradient → high energy → dominant process is vertical erosion (deepening the valley). V-shaped valleys, waterfalls, and gorges are the result.

Middle course: gentler gradient → both lateral erosion and deposition → meanders develop.

Lower course: lowest gradient → dominant process is deposition → floodplains and deltas are built.

Northern Ireland case study context

The River Bann (longest river in NI, draining the south from the Mourne Mountains to the north coast) and the River Lagan (draining into Belfast Lough) are used as case studies. CCEA examiners appreciate references to specific NI rivers alongside general process knowledge.