GCSE Geography (AQA 8035) — River Landscapes in the UK

Erosion processes

Rivers erode their bed and banks through four main processes.

Erosion provides the material that the river then transports and eventually deposits.

Erosion processes — Key Knowledge

Hydraulic action force of water dislodges particles from bed and banks
Abrasion rocks carried by the river scrape and wear away the bed and banks
Attrition rocks collide and become smaller and rounder downstream
Solution/corrosion river water dissolves soluble minerals from rocks

Transportation processes

Rivers move eroded material downstream by four methods, depending on particle size and river energy.

The method of transport depends on particle size and how much energy the river has.

Transportation processes — Key Knowledge

Traction large boulders rolled along the bed
Saltation smaller pebbles bounced along the bed
Suspension fine silt and clay carried within the water
Solution dissolved minerals carried invisibly in the water

Changes downstream

River characteristics change predictably from source to mouth, described by the Bradshaw model.

Velocity actually increases downstream even though the river appears calmer — less friction per unit of water in the wider, deeper channel.

Changes downstream — Key Knowledge

Channel width increases, Channel depth increases, Velocity increases channel is smoother and more efficient despite lower gradient
Discharge increases more tributaries join
Gradient decreases, Load particle size decreases attrition makes particles smaller and rounder
Load quantity increases

V-shaped valleys and interlocking spurs

In the upper course, vertical erosion dominates as the river cuts downward, creating steep-sided valleys.

These are the characteristic landforms of the upper course where the river has high energy for downward erosion but low energy for lateral erosion.

V-shaped valleys and interlocking spurs — Key Knowledge

V-shaped valley vertical erosion cuts down, weathering steepens valley sides
Interlocking spurs river winds around ridges of hard rock that interlock like teeth of a zip — river lacks energy to erode laterally

Waterfalls and gorges

Waterfalls form where a band of hard rock overlies softer rock, creating differential erosion.

Waterfall retreat is an ongoing process — the gorge downstream records how far the waterfall has moved.

Waterfalls and gorges — Key Knowledge

Soft rock erodes faster beneath the hard rock hydraulic action and abrasion
Overhang of hard rock collapses when unsupported, Plunge pool forms at the base, Waterfall retreats upstream over time leaving a steep-sided gorge, Example: High Force on the River Tees Whin Sill dolerite over limestone and shale

Meanders

In the middle course, lateral erosion becomes more important and the river develops pronounced bends.

The contrast between erosion on the outside bend and deposition on the inside bend is the key process driving meander formation and migration.

Meanders — Key Knowledge

Thalweg fastest flow on the outside of bends — erodes the bank forming a river cliff
Slip-off slope slowest flow on the inside — deposition creates a gently sloping point bar
Ox-bow lake meander neck narrows, river breaks through during a flood, deposition seals off the old loop creating a crescent-shaped lake

Floodplains and levees

Wide, flat areas of land either side of the river in the middle and lower course, with natural embankments along the banks.

Floodplains are fertile because of repeated alluvium deposition, which is why people build on them despite the flood risk.

Floodplains and levees — Key Knowledge

Floodplain formed by lateral erosion meanders migrate, widening the valley floor) and deposition of alluvium during floods, Levees (natural embankments — river overflows and deposits heaviest material closest to the channel, repeated floods build them up
Estuaries where river meets the sea — wide shallow channels with mudflats

Causes of flooding

Flooding results from a combination of physical and human factors that increase the volume or speed of water reaching the river.

Most floods result from a combination of physical and human causes rather than a single factor.

Causes of flooding — Key Knowledge

Prolonged or intense rainfall saturates ground, causes surface runoff
Snowmelt, Impermeable rock e.g. granite, clay
Steep slopes increase runoff speed
Urbanisation impermeable surfaces — water reaches rivers faster via drains
Deforestation removes interception, increases surface runoff

Hydrographs

A flood hydrograph shows how river discharge changes over time after a rainfall event.

Lag time is the key measure — a short lag time means water reaches the river quickly and flooding is more likely.

Hydrographs — Key Knowledge

Rising limb discharge increasing
Peak discharge highest point
Falling/recession limb discharge decreasing
Lag time gap between peak rainfall and peak discharge
Short lag time = high flood risk impermeable surfaces, steep slopes, urbanisation, lack of vegetation
Long lag time = lower flood risk permeable rock, gentle slopes, dense vegetation

Flood management

Flood management strategies are divided into hard engineering (physical structures) and soft engineering (working with natural processes).

Hard engineering is often more effective short-term but more expensive; soft engineering is cheaper and more sustainable but slower to take effect.

Flood management — Key Knowledge

Hard — dams and reservoirs store water upstream, expensive but provide HEP and water supply
Hard — embankments/flood walls raise banks artificially
Hard — channel straightening speeds flow but increases risk downstream
Hard — flood relief channels divert water during high discharge
Soft — floodplain zoning restrict building on floodplains
Soft — afforestation trees intercept rainfall and slow runoff, takes years
Soft — river restoration returning river to natural course
Soft — managed flooding/washlands deliberate flooding of low-value land to protect settlements

River Landscapes in the UK

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River Landscapes in the UK