River Processes and Landforms

The River's Work: Erosion, Transportation and Deposition

Rivers are agents of erosion, transportation and deposition. These three processes shape the landscape from the source (upland) to the mouth (lowland/sea).

Erosion — Four Types

Rivers erode in four ways:

1. Hydraulic action: The sheer force of moving water against the river bank/bed. Most powerful when the river is in flood.
2. Abrasion (corrasion): Rock particles carried by the river scrape and sand-paper the bed and banks — the most effective form of erosion.
3. Attrition: Rock particles knock against each other, becoming smaller and rounder as they travel downstream.
4. Corrosion (solution): River water dissolves soluble rock (e.g., limestone). The water becomes slightly acidic from absorbed CO₂.

Transportation — Four Types

Rivers carry sediment in four ways:

1. Traction: Large boulders rolled along the riverbed.
2. Saltation: Pebbles bounced along the bed in a "leaping" motion.
3. Suspension: Fine particles (silt, clay) carried within the flow — gives rivers their brown/murky colour.
4. Solution: Dissolved minerals carried invisibly in the water.

The capacity of a river is how much it can carry; the competence is the maximum size of particle it can move. Both increase dramatically with velocity — the Hjulström curve shows that a small increase in velocity enables a river to carry much larger particles.

Deposition

Rivers deposit sediment when they slow down (e.g., entering a lake, on the inside of a bend, in a flood). Heaviest particles drop first; lightest (clay) last.

Landforms in the Upper Course

V-shaped Valley

• Formed by downcutting (vertical erosion) — the river cuts downward faster than the sides are worn away
• Weathering of the steep sides causes material to fall into the river
• Results in a steep, narrow, V-shaped valley — the river is confined within it

Waterfall

1. River flows over bands of hard and soft rock
2. Soft rock erodes faster → creates a step
3. Hydraulic action creates a plunge pool at the base
4. Rock above the plunge pool overhangs and eventually collapses
5. Waterfall retreats upstream over time, leaving a gorge

Example: High Force Waterfall, Teesdale (whinstone over limestone)

Landforms in the Middle/Lower Course

Meander

• Formed by lateral erosion (sideways erosion) as the river swings left and right
• Erosion on the outer bank (fast-flowing) → creates a river cliff
• Deposition on the inner bank (slow-flowing) → creates a slip-off slope (point bar)
• The channel gradually becomes more sinuous (curvy)

Oxbow Lake

1. A very tight meander forms — the neck of land between loops becomes very narrow
2. In flood, the river cuts across the neck (hydraulic action)
3. The old meander loop is cut off
4. Deposition seals the entrance → an oxbow lake forms
5. Over time, it may be colonised by vegetation and dry out

Floodplain

• Wide, flat valley floor built up by repeated flooding
• Rivers deposit alluvium (fine silt) each time they flood
• The most fertile agricultural land in the UK — e.g., the Thames floodplain, the Vale of York
• Levées (natural raised banks) may form on the edges of the channel from coarser deposition

WJEC Exam Tips

• Always use the correct technical vocabulary for processes and landforms
• For 6-mark extended answer questions, use a labelled diagram if asked; describe stages in order
• Case studies: you need at least one UK river (e.g., River Tees, River Exe) for management questions
• Connect processes to landforms: a question about a waterfall requires you to describe the process of erosion that creates it