GCSE geography Rivers

Drainage basin

Area of land drained by a river and its tributaries

Watershed

Boundary of a drainage basin usually formed by hills or mountains

Tributary

A small river that joins a larger river

Confluence

Point where two rivers meet

River source

The starting point of a river

River mouth

The point where a river meets the sea

Upper course

Steep gradient, narrow channel, vertical erosion, low velocity

Middle course

Wider channel, increased discharge, lateral erosion and deposition

Lower course

Wide deep channel, high discharge, deposition dominant

River velocity

The speed at which water flows in a river

River friction

Contact between water and river bed slowing the flow

Velocity upper course

Low velocity due to shallow channel and high friction

Velocity lower course

Higher velocity due to deeper channel and less friction

Vertical erosion

Erosion downwards into the river bed

Lateral erosion

Erosion sideways into the river banks

Abrasion

Rocks carried by river scrape the bed and banks

Attrition

Rocks collide and become smaller and rounder

Hydraulic action

Force of water breaks rock from river banks

Solution

River water dissolves soluble rock such as limestone

River load

Material transported by a river

Transportation solution

Chemicals dissolved in river water

Transportation suspension

Fine material carried within the water

Transportation saltation

Small pebbles bounce along the river bed

Transportation traction

Large rocks roll along the river bed

Deposition

Dropping of river load when velocity decreases

Deposition location

Most deposition occurs in the lower course

Upper course landforms

V-shaped valleys, interlocking spurs, waterfalls

Middle course landforms

Meanders and gorges

Lower course landforms

Floodplains, levees, ox-bow lakes, estuaries

Interlocking spurs

River winds around hard rock in upper course

V-shaped valley

Steep-sided valley formed by vertical erosion

Waterfall

Forms where river flows over hard and soft rock

Hard rock

Rock resistant to erosion

Soft rock

Rock that erodes easily

Plunge pool

Deep hollow at base of waterfall

Waterfall retreat

Waterfall moves upstream over time

Gorge

Steep-sided valley left behind by retreating waterfall

Meander

Bend in a river channel found in middle course

River cliff

Steep bank on outside of meander caused by erosion

Slip-off slope

Gently sloping bank on inside of meander caused by deposition

Thalweg

Path of fastest flowing water in a river

Ox-bow lake

U-shaped lake formed from cut-off meander

Ox-bow formation

Erosion narrows neck then flood cuts through

Floodplain

Flat land beside river formed by deposition

Alluvium

Fine fertile sediment deposited on floodplains

Levees

Raised river banks formed by repeated flooding

Levee formation

Sediment deposited along banks during floods

Estuary

Mouth of river influenced by tides

Mudflats

Depositional landforms in estuaries

Flooding

When river exceeds channel capacity

Floodplain flooding

Worst flooding occurs on floodplains

Human causes of flooding

Urbanisation and deforestation

Urbanisation

Increases impermeable surfaces and runoff

Deforestation

Reduces interception increasing runoff

Physical causes of flooding

Heavy rain, impermeable rock, steep relief

Impermeable rock

Rainwater runs over surface increasing runoff

Topography effect

Steep slopes increase speed of runoff

River capacity

Blockages reduce capacity increasing flood risk

Storm hydrograph

Graph showing river discharge after rainfall

Discharge

Volume of water flowing in a river

Peak rainfall

Maximum rainfall on hydrograph

Rising limb

Increase in discharge after rainfall

Peak flow

Highest discharge level

Lag time

Time between peak rainfall and peak discharge

Falling limb

Decrease in discharge after peak flow

Base flow

Normal river discharge

Flashy hydrograph

Short lag time, high peak flow

Subdued hydrograph

Long lag time, low peak flow

Flash flood

Rapid flooding caused by intense rainfall

Subdued flood

Slow flooding caused by prolonged rainfall

Hydrograph factors

Rainfall, geology, relief, vegetation, basin size

Vegetation effect

Interception reduces runoff

Small drainage basin

Creates flashy hydrograph

Large drainage basin

Creates subdued hydrograph

Drainage basin management

Managing rivers to reduce flood risk

Purpose of management

Protect people, property and environment

Sustainable management

Meets present needs without harming future

Soft engineering

Natural methods to manage flooding

Floodplain zoning

Building away from rivers

Floodplain zoning advantage

Reduces damage without altering river

Floodplain zoning disadvantage

Only works for new developments

Wetlands

Natural floodwater storage areas

Wetlands advantage

Create habitats and reduce flooding

Wetlands disadvantage

Take time to establish

Hard engineering

Man-made structures to control rivers

Dam

Concrete barrier storing water

Dam advantages

Controls flow and generates electricity

Dam disadvantages

Floods land and damages ecosystems

Embankments

Raised river banks

Embankment advantage

Protects nearby property

Embankment disadvantage

Destroys river habitats

Dredging

Removing sediment from river bed

Dredging advantage

Increases channel capacity

Dredging disadvantage

Requires regular maintenance

Channel straightening

Straightens river to increase speed

Channel straightening advantage

Reduces flood risk

Channel straightening disadvantage

Increases erosion downstream

Flood preparation

Sandbags, barriers and warnings

Environment Agency

Monitors rivers and issues flood warnings