Edexcel GCSE Geography B (1GB0) — UK Physical Landscapes
Every time there is a massive thunderstorm, a river's energy and behaviour change completely compared to a dry summer day. High precipitation intensity increases river discharge, providing more energy. This extra energy drives vertical erosion in the steep upper course and lateral erosion in the middle and lower courses.
Conversely, high temperatures increase evapotranspiration, which reduces discharge and limits the river's capacity to transport its sediment load. In cold climates, low temperatures increase the frequency of freeze-thaw weathering on valley sides. This physical weathering shatters rock, adding angular material (scree) directly into the river channel.
You might think water shapes the landscape entirely on its own, but the underlying rock actually controls the river's power. Hard, resistant rocks (like Granite and Slate) erode slowly, typically forming steep-sided V-shaped valleys and waterfalls. Soft, unconsolidated rocks (like Clay and Shale) erode easily, creating wider valleys and contributing a larger volume of fine sediment to the river.
Permeability is equally crucial for controlling flood risk. Permeable rocks (like Chalk and Limestone) allow rainwater to soak into the ground via infiltration and percolation. This slows down the water, lengthening the lag-time. Impermeable rocks (like Slate and Clay) prevent infiltration. This forces water to flow rapidly over the surface as surface runoff (or overland flow), leading to much higher discharge peaks.
If you have ever seen a landslide or rockfall, you have witnessed gravity directly shaping a river valley. Unlike fluvial erosion which requires flowing river water, mass movement only requires a slope and gravity to transport material. While vertical erosion deepens the river channel, the widening of a V-shaped valley is actually driven by mass movement on the valley sides.
Any slope with a gradient greater than is susceptible to this downward movement. These processes supply the vast majority of the river's sediment load. The main types include:
The river transports this weathered material in four ways, depending on its energy: traction (rolling large boulders), saltation (bouncing small pebbles), suspension (carrying fine particles within the water column), and solution (carrying dissolved minerals). Hard rocks like granite contribute to the bedload via rockfalls, while soft rocks like clay contribute fine suspended sediment via slumping. This fine sediment is eventually deposited lower down the river as alluvium.
Predicting exactly when a river will burst its banks is a matter of life and death for communities living on floodplains. A storm hydrograph is a combined line graph and bar chart showing how a river responds to a specific rainfall event. The bar chart represents rainfall, while the line graph represents discharge.
The volume of water flowing in the river is calculated using the following equation:
Where:
The steepness of the rising limb on a hydrograph shows how quickly rainwater enters the channel. The highest volume of water reached is the peak discharge. If this peak exceeds the bankfull discharge, the river will spill over its banks and flood.
Why do two river basins receiving the exact same amount of rain respond completely differently? Physical characteristics of the drainage basin determine whether a hydrograph is 'flashy' (high flood risk) or 'subdued' (low flood risk).
| Physical Factor | Flashy Hydrograph (Short Lag-Time) | Subdued Hydrograph (Long Lag-Time) |
|---|---|---|
| Relief | Steep-sided valleys move water rapidly. | Gentle slopes allow time for infiltration. |
| Geology | Impermeable rocks prevent soaking in. | Permeable rocks allow percolation. |
| Basin shape | Circular basins deliver water simultaneously. | Elongated basins stagger water arrival. |
| Antecedent moisture | Saturated soil prevents infiltration. | Dry soil allows rainwater to soak in. |
| Vegetation | Sparse cover; low interception. | Dense cover; high interception/evapotranspiration. |
Lag-time is calculated using the formula:
A storm begins at 08:00. The centre of the highest rainfall bar is recorded at 12:00. The highest point on the discharge line occurs at 17:30. Calculate the lag-time.
Step 1: Identify the values.
Step 2: Substitute into the equation.
Students often confuse 'river regime' (the annual pattern of discharge) with a 'storm hydrograph' (a river's response to a single storm event). Ensure you identify the correct timeframe in the question.
In 2-mark or 3-mark explanation questions, examiners expect you to make clear causal links: state the factor (e.g., impermeable granite), explain the mechanism (prevents infiltration), and give the outcome (increases surface runoff and shortens lag-time).
When calculating lag-time from a hydrograph, always measure from the exact centre of the peak rainfall bar to the highest point of the peak discharge line.
When explaining the formation of a V-shaped valley, remember to mention that while vertical erosion deepens the channel, it is weathering and mass movement that actually widen the valley sides into a 'V' shape.
Discharge
The volume of water passing a specific point in a river at a given time, measured in cubic metres per second (cumecs).
Vertical erosion
Downward erosion that deepens a river channel, most common in the upper course.
Lateral erosion
Sideways erosion that widens a river channel and valley, most common in the middle and lower courses.
Sediment load
The total volume of material, including rocks, sand, clay, and dissolved minerals, transported by a river.
Freeze-thaw weathering
A physical weathering process where water freezes, expands, and shatters rock in cold climates.
Weathering
The breakdown or decay of rocks in situ (in their original place) by physical, chemical, or biological processes.
Permeability
The ability of a rock or soil to allow water to pass through it.
Infiltration
The process by which water on the ground surface enters the soil.
Lag-time
The time delay between peak rainfall and peak discharge on a storm hydrograph.
Surface runoff
The rapid movement of water over the ground surface when the soil is saturated or the rock is impermeable.
Mass movement
The downhill movement of weathered material (rock and soil) under the influence of gravity.
Soil creep
The very slow downhill movement of soil particles caused by expansion and contraction.
Slumping
A type of mass movement where saturated soil or weak rock slides downslope along a curved surface.
Rockfalls
The rapid free-fall of rock from a steep cliff face, often triggered by freeze-thaw weathering.
Traction
A method of river transport where large boulders and rocks are rolled along the riverbed.
Saltation
A method of river transport where small pebbles and stones bounce or leap along the riverbed.
Suspension
A method of river transport where very fine particles like silt and clay are carried within the water column.
Solution
A method of river transport where dissolved minerals are carried invisibly in the water.
Alluvium
Fine sediment, such as silt and clay, deposited by a river on its floodplain.
Storm hydrograph
A graph showing the change in a river's discharge over a short period following a specific rainfall event.
Rising limb
The segment of a storm hydrograph showing the increase in discharge as rainwater enters the river channel.
Peak discharge
The maximum volume of water flowing in a river during a storm event.
Bankfull discharge
The maximum discharge a river channel can carry before it spills over its banks and floods.
Antecedent moisture
Water already present in the soil from previous rainfall, which determines the level of soil saturation.
Evapotranspiration
The combined loss of water from the Earth's surface through evaporation and from plants through transpiration.
Percolation
The downward movement of water through the pores or cracks in rock towards the water table.
Put your knowledge into practice — try past paper questions for Geography B
Discharge
The volume of water passing a specific point in a river at a given time, measured in cubic metres per second (cumecs).
Vertical erosion
Downward erosion that deepens a river channel, most common in the upper course.
Lateral erosion
Sideways erosion that widens a river channel and valley, most common in the middle and lower courses.
Sediment load
The total volume of material, including rocks, sand, clay, and dissolved minerals, transported by a river.
Freeze-thaw weathering
A physical weathering process where water freezes, expands, and shatters rock in cold climates.
Weathering
The breakdown or decay of rocks in situ (in their original place) by physical, chemical, or biological processes.
Permeability
The ability of a rock or soil to allow water to pass through it.
Infiltration
The process by which water on the ground surface enters the soil.
Lag-time
The time delay between peak rainfall and peak discharge on a storm hydrograph.
Surface runoff
The rapid movement of water over the ground surface when the soil is saturated or the rock is impermeable.
Mass movement
The downhill movement of weathered material (rock and soil) under the influence of gravity.
Soil creep
The very slow downhill movement of soil particles caused by expansion and contraction.
Slumping
A type of mass movement where saturated soil or weak rock slides downslope along a curved surface.
Rockfalls
The rapid free-fall of rock from a steep cliff face, often triggered by freeze-thaw weathering.
Traction
A method of river transport where large boulders and rocks are rolled along the riverbed.
Saltation
A method of river transport where small pebbles and stones bounce or leap along the riverbed.
Suspension
A method of river transport where very fine particles like silt and clay are carried within the water column.
Solution
A method of river transport where dissolved minerals are carried invisibly in the water.
Alluvium
Fine sediment, such as silt and clay, deposited by a river on its floodplain.
Storm hydrograph
A graph showing the change in a river's discharge over a short period following a specific rainfall event.
Rising limb
The segment of a storm hydrograph showing the increase in discharge as rainwater enters the river channel.
Peak discharge
The maximum volume of water flowing in a river during a storm event.
Bankfull discharge
The maximum discharge a river channel can carry before it spills over its banks and floods.
Antecedent moisture
Water already present in the soil from previous rainfall, which determines the level of soil saturation.
Evapotranspiration
The combined loss of water from the Earth's surface through evaporation and from plants through transpiration.
Percolation
The downward movement of water through the pores or cracks in rock towards the water table.
