Rivers
Map Your Gaps
Card 1 of 12
Swipe right if you know it, left if you don't
✔ Know
✖ Don't know
Drainage basin terms
A drainage basin is the area of land drained by a river and its tributaries.
Every drop of rain that falls within a drainage basin will eventually flow into the main river.
Drainage basin terms — Key Knowledge
- Drainage basin area of land drained by a river system
- Watershed the boundary of a drainage basin — usually a ridge of high land
- Source where a river begins
- Mouth where a river meets the sea or a lake
- Tributary a smaller river that joins a larger one
- Confluence the point where two rivers meet
Hydrological cycle in a drainage basin
Water moves through a drainage basin by several different pathways.
The speed water reaches the river depends on which pathway it takes — overland flow is fastest, groundwater flow is slowest.
Hydrological cycle in a drainage basin — Key Knowledge
- Precipitation rain, snow, sleet, hail
- Interception vegetation catches rainfall before it reaches the ground
- Infiltration water soaks into the soil
- Throughflow water moves through the soil downhill
- Groundwater flow water moves slowly through underground rock
- Overland flow surface runoff — water flows over the ground when soil is saturated or impermeable
- Evaporation water turns to vapour from surfaces
- Transpiration water lost from plants through leaves
River discharge
The volume of water passing a point in a river per second.
After heavy rain, discharge rises — if it rises too high, the river floods.
River discharge — Key Knowledge
- Discharge volume of water per second, measured in cumecs — m³/s
- Increases downstream more tributaries add water
- Affected by rainfall, rock type, land use, season
Erosion processes
Four processes by which a river wears away its bed and banks.
Erosion is strongest when the river has high velocity and high discharge — typically after heavy rain.
Erosion processes — Key Knowledge
- Hydraulic action force of water pushing into cracks in the rock, compressing air and breaking rock apart
- Abrasion/corrasion rocks carried by the river scrape and wear away the bed and banks
- Attrition rocks bump into each other and break into smaller, rounder pieces
- Solution/corrosion slightly acidic river water dissolves soluble rock such as limestone
Transport processes
Four ways a river moves its load (the material it carries).
The faster the river flows, the larger the material it can transport.
Transport processes — Key Knowledge
- Traction large boulders rolled along the river bed
- Saltation smaller stones bounced along the bed
- Suspension fine particles carried within the water — makes it look muddy
- Solution dissolved minerals carried invisibly in the water
Deposition
When a river loses energy, it drops the material it is carrying.
Deposition builds features like flood plains, levées and deltas.
Deposition — Key Knowledge
- Deposition occurs when river slows down — inside of a meander, river enters a lake or sea, river floods onto flat land, shallow water
- Largest material deposited first heaviest particles dropped when energy falls
Upper course landforms
Near its source, a river is steep, narrow and fast-flowing with mainly vertical erosion.
Waterfalls retreat upstream — the gorge shows where the waterfall used to be.
Upper course landforms — Key Knowledge
- V-shaped valley vertical erosion deepens the channel; weathering and mass movement widen the valley sides
- Interlocking spurs river winds around ridges of hard rock — spurs overlap like interlocking fingers
- Waterfalls hard rock overlies soft rock; soft rock erodes faster creating an overhang; plunge pool forms at base; overhang collapses; waterfall retreats upstream leaving a gorge
Middle and lower course landforms
Further downstream, the river is wider and slower with more lateral (sideways) erosion.
The river's cross-section gets wider and shallower as you move downstream.
Middle and lower course landforms — Key Knowledge
- Meanders river bends — erosion on the outside bend where flow is fastest, deposition on the inside bend where flow is slowest; river cliff on outside, slip-off slope on inside
- Oxbow lakes extreme meander neck narrows; river breaks through during flood; deposition seals off the old loop; abandoned meander becomes an oxbow lake
- Flood plain wide, flat area of land either side of a river — formed by deposition during floods
- Levées natural raised banks along the river — built up by repeated deposition of coarser material during floods
Deltas
A delta forms where a river meets the sea or a lake and deposits its load.
Famous deltas include the Nile, Ganges and Mississippi.
Deltas — Key Knowledge
- Forms when river slows as it enters the sea, deposits sediment
- Conditions needed large sediment load, calm/shallow water, weak tidal currents
- River splits into distributaries — smaller channels spreading across the delta
Causes of flooding
Rivers flood when discharge exceeds the capacity of the channel.
Flooding is often caused by a combination of factors, not just one.
Causes of flooding — Key Knowledge
- Prolonged or intense rainfall soil becomes saturated
- Impermeable surfaces tarmac, concrete — common in urban areas
- Deforestation less interception, faster surface runoff
- Steep slopes water reaches the river quickly
- Snowmelt rapid thaw adds large volumes of water
- Narrow or shallow channels limited capacity
Flood management
Strategies to reduce the impact of river flooding — divided into hard and soft engineering.
Hard engineering is expensive but effective; soft engineering is cheaper and more sustainable but may not prevent all flooding.
Flood management — Key Knowledge
- Hard engineering dams — control flow and store water; levées — raised banks to contain the river; channel straightening — speeds up flow to move water away faster; flood walls — protect specific areas
- Soft engineering flood plain zoning — restricting building on flood-risk land; afforestation — planting trees to increase interception; flood warning systems — give time to evacuate; wetland restoration — natural flood storage
Case study required
The spec requires a named river case study.
The case study should cover opportunities, hazards and management — not just flooding.
Case study required — Key Knowledge
- The opportunities presented by a river or rivers water supply, fertile land, transport, tourism
- The associated hazards flooding, erosion
- Their management specific strategies used