River Processes and Landforms

Rivers shape their valleys and channels through four erosion processes, four transportation modes, and deposition. These processes operate simultaneously but with different intensity depending on where the river is in its long profile.

Erosion processes:

Process	Mechanism	Where dominant
Hydraulic action	The force of the water itself; compressed air in cracks explodes as water retreats, widening joints and fractures	Upper course waterfalls and rapids; meander outer banks
Abrasion (corrasion)	Sediment carried by the river scrapes and grinds the channel bed and banks	Wherever the river carries coarse sediment
Attrition	Rocks and pebbles carried by the river collide with each other, becoming progressively smaller, rounder, and smoother	Throughout the river; pebbles reduce from angular blocks to smooth, rounded gravel
Solution (corrosion)	Slightly acidic water dissolves soluble minerals in rock (e.g. calcium carbonate in limestone); material removed in chemical solution	Limestone areas; chalk catchments

Transportation modes:

Mode	What is carried	Condition needed
Solution	Dissolved minerals	Constant — occurring continuously in rivers flowing over soluble rock
Suspension	Fine clay, silt, and sand particles	Moderate to high velocity; gives rivers a brown/murky colour in flood
Saltation	Sand and small pebbles bouncing along the bed	Moderate velocity
Traction	Large boulders and cobbles rolled along the bed	High velocity and high discharge

Deposition occurs when river velocity falls — most commonly when discharge decreases, when a river meets a lake or the sea, or on the inside of a bend where velocity is lower.

A river's long profile plots the gradient (steepness) of the riverbed from source to mouth. It is typically concave — steep in the upper course and more gradual in the lower course — as the river works to achieve a smooth equilibrium gradient called the graded profile. The diagram below shows the concave shape and where each course sits along it.

Changes along the long profile:

Characteristic	Upper course	Middle course	Lower course
Gradient	Steep	Moderate	Gentle
River velocity	Slow (rough, narrow channel)	Moderate	Fastest (smooth, wide channel)
Discharge	Low	Increasing	Highest
Channel width and depth	Narrow and shallow	Wider and deeper	Widest and deepest
Sediment size	Large, angular boulders	Mixed: cobbles to gravel	Fine sand, silt, and clay
Dominant process	Vertical (downward) erosion	Lateral (sideways) erosion and transportation	Transportation and deposition
Typical landforms	Interlocking spurs, V-shaped valley, waterfalls	Meanders beginning	Meanders, floodplains, levées, oxbow lakes

An important misconception to correct: river velocity does not decrease from source to mouth. Although the gradient flattens, the channel becomes much smoother, wider, and deeper downstream, reducing friction — so velocity actually increases overall despite the gentler gradient. Discharge increases as tributaries add water.

V-shaped valleys and interlocking spurs: In the upper course, vertical erosion cuts downward, forming a steep-sided V-shaped valley. Hard rock ridges called interlocking spurs alternate on either side of the valley, forcing the river to wind between them. The valley sides are steep because rainwater and mass movement continually add material to the river, which cuts deeper rather than widening its channel.

Waterfalls and gorges:

1. A river flows over a band of resistant rock (e.g. dolerite, sandstone) overlying softer rock (e.g. shale, mudstone)
2. The softer rock below erodes more quickly; a step forms
3. Hydraulic action and abrasion cut a plunge pool at the base of the fall through the force of the falling water and swirling boulders (pothole drilling)
4. The softer rock undercuts the hard cap rock, leaving it overhanging
5. The overhang collapses (undercutting + crack widening); the waterfall retreats upstream
6. A gorge of recession marks the previous positions of the waterfall — a steep-sided, narrow valley cut by the retreating fall

The cross-section below shows the key features examiners look for: the resistant cap rock, the undercut overhang, the plunge pool, and the gorge left behind as the fall retreats upstream.

UK example — High Force, Upper Teesdale (County Durham):

• One of England's largest waterfalls (21 m drop)
• The Whin Sill (resistant igneous dolerite) overlies soft Carboniferous limestone/shale
• The gorge behind High Force is approximately 500 m long, representing the distance the waterfall has retreated since the last ice age
• The plunge pool is clearly visible at low flow

Meander formation:

1. Even in a straight channel, variations in velocity cause small irregularities; water moves faster on one side than the other
2. The faster (outer) bank experiences hydraulic action and abrasion — a river cliff forms; the slower (inner) bank deposits sediment as a point bar (slip-off slope)
3. Erosion of the outer bank and deposition on the inner bank gradually increase the curve
4. As the meander tightens, the loop becomes more pronounced

Oxbow lake formation:

1. A meander becomes very tightly curved (almost a complete loop)
2. The narrow neck of land between the two sides of the loop is eroded through (during a flood when velocity and discharge are high)
3. The river cuts straight across the neck — the main channel takes the shortest, steepest path
4. The old meander loop is sealed off by deposition at each end
5. The abandoned loop becomes an oxbow lake (or moraine lake), which gradually silts up and becomes a marshy wetland over decades

Floodplain: The wide, flat valley floor on either side of the river channel in the lower course. It is built up by the repeated deposition of fine alluvial sediment during floods. Each flood adds another layer of fertile alluvium. Floodplains are:

• Used extensively for agriculture (flat, fertile, easy to cultivate)
• Subject to repeated flooding — this is their natural state, not a hazard per se
• Increasingly built on by urban areas, which increases flood risk for properties

Levées: Natural raised banks of sediment on either side of the channel in the lower course:

1. During a flood, the river overtops its banks
2. At the point of overtopping, velocity immediately decreases (the river is no longer confined to its channel)
3. The coarsest sediment is deposited first, right next to the bank
4. Repeated floods build up a natural embankment — the levée
5. Levées can make flooding worse by raising the river level until the levée is overtopped catastrophically

Estuaries: An estuary is the wide, funnel-shaped mouth of a river where it meets the sea and is influenced by tidal action. Estuaries form where the tidal range is sufficient to prevent sediment from building up across the mouth (distinguishing them from deltas, which form only where wave and tidal energy are low). Characteristics:

• River water mixes with salt water — brackish water zone
• Tides bring sediment up the estuary at flood tide and remove it at ebb tide; fine silt and clay settle in sheltered conditions, forming mud flats and salt marshes on the margins
• Estuaries are highly productive ecosystems; the rich mudflats support large populations of wading birds and fish nurseries
• Major UK estuaries: Thames Estuary (London to the North Sea), Severn Estuary (between England and Wales; the world's second-highest tidal range at up to 14 m), Humber Estuary (Hull; formed by the combined Rivers Ouse and Trent)

Deltas (extra context — beyond AQA 8035 spec): Form where a river meets a sea with very low wave energy — sediment is deposited faster than it can be removed. The river splits into distributaries; sediment fans out and builds new land. Example: Nile Delta (Egypt).

The River Tees provides a single example spanning the full range of erosion and deposition landforms from source to mouth, making it a well-suited UK river valley case study.

Source to mouth overview:

• Source: Cross Fell, Pennines (893 m); flows eastward for 137 km to the North Sea near Middlesbrough
• Upper course: steep gradients, impermeable gritstone and limestone, rapid vertical erosion
• Lower course: widens across the Vale of York and Teesdale lowlands; extensive floodplains and meanders before reaching the Tees Estuary

Key landforms along the Tees:

Location	Landform	Notes
High Force, Upper Teesdale	Waterfall (21 m) and gorge	Whin Sill dolerite overlies softer limestone/shale; plunge pool clearly visible; gorge ~500 m long marks the retreat of the fall
Low Force, Upper Teesdale	Series of rapids and small falls	Stepped profile as river crosses Whin Sill outcrops
Cauldron Snout, Upper Teesdale	Cascade (UK's longest waterfall by flow length)	River drops approximately 60 m over resistant whinstone (Whin Sill)
Middle Teesdale	Interlocking spurs and V-shaped valley	Hard Carboniferous limestone ridges constrain the valley
Lower Teesdale / Teesside	Meanders, floodplains, levées	Wide alluvial floodplain; natural and artificial levées along the lower reaches
Tees Estuary	Estuary with mud flats and salt marsh	Tidal influence extends several km inland; RSPB Teesmouth reserve on estuary mud flats

The Tees illustrates how a single river can display the complete sequence of fluvial landforms within one catchment — from the dramatic waterfall and gorge scenery of the upper course through to the broad floodplain and estuary of the lower course.

1. Stating that rivers flow faster in the upper course

River velocity is actually highest in the lower course, where the channel is wide, deep, and smooth (low friction). In the upper course, the channel is narrow, shallow, and rough (high friction), so water moves slowly despite the steep gradient. Discharge also increases downstream as tributaries join.

2. Describing waterfall retreat without mentioning undercutting

"The waterfall erodes backwards" earns one mark. The complete sequence — softer rock erodes faster creating undercutting, overhang collapses, waterfall retreats upstream leaving a gorge — earns many more. Name the hard and soft rock layers in a named example (e.g. Whin Sill dolerite over limestone shale at High Force).

3. Confusing deposition on the inside of a bend with the outside

Deposition (slip-off slope/point bar) occurs on the inside of a meander — velocity is lowest there. Erosion (river cliff) occurs on the outside — velocity is highest. Drawing this wrong in a diagram will lose marks.

4. Describing oxbow lakes as forming from any meander

Oxbow lakes form specifically when the neck of a very tightly curved meander is cut through during a flood. A newly formed meander is not going to become an oxbow lake soon. The key trigger is flood conditions + a very narrow neck, not just the presence of a meander.

5. Forgetting that floodplains are built by deposition, not only erosion

Students sometimes focus only on lateral erosion widening the valley floor. Floodplains are also built upward by the repeated deposition of alluvium during floods. This is why floodplain soils are so fertile — they are composed of nutrient-rich silt deposited over thousands of years.