🌧 Prolonged or intense rainfall
Days of rain, or a sudden downpour, put more precipitation into the basin than the soil can take in. The extra water becomes overland flow and rushes into the channel.
Rivers: Opportunities and Hazards » Why Rivers Flood and What Floods Do
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 1.3.2, 1.3.3
- The natural and human causes of river flooding
- How each cause makes water reach the river faster
- Social, economic and environmental impacts, good and bad
- How flood impacts differ between LICs and HICs
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Unlock This CourseWhen does a river flood?
A river floods when it holds more water than its channel can carry, so water spills over the banks onto the land beside it. This happens when a lot of water reaches the river in a short time. The faster water moves across or through the ground, the sooner the river rises. A hydrograph is the graph geographers use to show how a river responds to rain, and you will meet it later in the course.
Some causes of flooding are natural. Others are caused by people. Almost every cause works in one of two ways: it puts more water into the basin, or it makes water reach the channel faster. When you explain a cause in the exam, say which one it is.
Key terms:
- Flood: when a river overflows its banks and covers land that is normally dry.
- Runoff: water that flows over the ground surface towards a river.
- Snowmelt: water produced when snow and ice melt.
- Urbanisation: the growth of towns and cities.
- Deforestation: the cutting down of trees over a large area.
Natural causes of flooding
❄ Snowmelt
In spring, or in a warm spell, a big store of snow melts quickly. All that water flows into rivers at once, especially in mountain basins.
💦 Saturated ground
When the soil is already full of water, there is no room left. Infiltration stops and nearly all new rain runs off as overland flow.
🧊 Frozen ground
Frozen soil acts like a hard lid. Water cannot soak in, so it runs straight over the surface.
🧱 Impermeable rock
Rock such as clay does not let water pass through. There is little percolation, so water stays on the surface and flows quickly into rivers.
⛰ Steep slopes
Water runs downhill fast on steep land. It has less time to soak in, so overland flow and throughflow reach the channel quickly.
🌀 Tropical storms
These bring very heavy rain and strong winds, often over a large area. Rivers can rise within hours.
Human causes of flooding
People do not make it rain, but we can make floods more likely or worse.
| Human cause | How it makes flooding more likely |
|---|---|
| Urbanisation and impermeable surfaces | Roads, roofs and car parks are impermeable. There is no infiltration, so rain runs into drains and then into rivers very quickly. |
| Deforestation | Fewer trees means less interception and less transpiration. More rain hits the soil, which fills up and runs off. |
| Farming practices | Bare soil, overgrazing and ploughing up and down a slope reduce plant cover and compact the soil. Water runs off instead of soaking in. |
| Building on floodplains | This does not add water, but it puts more people and buildings in the path of a flood. |
| Climate change | A warmer atmosphere can hold more water, so some places get heavier rainfall. It can also change snowmelt patterns. |
Common mistakes
Do not just write "heavy rain causes floods". Say why: the water cannot infiltrate, so overland flow rises and reaches the channel quickly. Also remember that building on a floodplain increases the damage floods cause, not the amount of water.
The impacts of flooding
An impact is an effect of a flood. Geographers sort impacts into three groups.
- Social: effects on people's lives, such as deaths, injuries, homelessness and disease.
- Economic: effects on money and jobs, such as damaged homes, businesses and roads, and the cost of repairs.
- Environmental: effects on the natural world, such as wildlife, soil and water.
| Negative | Positive | |
|---|---|---|
| Social | People drown or are injured; homes destroyed; dirty water spreads disease; stress | Communities may come together to help each other |
| Economic | Homes, shops, crops and roads damaged; businesses close; jobs lost; repair bills | Rebuilding creates work; richer soil can raise later harvests |
| Environmental | Animals drowned; habitats damaged; soil washed away; sewage and chemicals spread | Fresh silt makes fertile soil; groundwater is recharged |
Key terms:
- Silt: fine, fertile mud left on the land when a flood goes down.
- Recharge: when water soaks down and refills the groundwater store.
Impacts at different scales
A flood can matter at more than one scale. Locally, a family loses its home. Regionally, a whole river valley loses crops, roads and electricity. Nationally, a government has to spend a lot on repairs and aid. In the biggest events the effects can spread globally: damaged farmland can reduce food exports and push up prices elsewhere.
LICs and HICs: the same flood, different results
🏚 In LICs and poorer MICs
More people live in poor housing on floodplains and farm for a living. Warning systems and rescue services are limited. Deaths and disease are often higher, and recovery can take years. In 2022, monsoon floods in Pakistan, a lower-middle-income country, killed more than 1,700 people and affected about 33 million.
🏢 In HICs
Fewer people die because of better warnings, stronger buildings and faster rescue. But the money cost is often very high because homes and businesses are worth more. In July 2021, floods in western Germany killed at least 184 people and destroyed homes, bridges and roads.
This means a flood of the same size can cause more human suffering in an LIC or poorer MIC, and more financial damage in an HIC.
Exam-style question
Fig. 1.1 describes two made-up river basins after the same storm. Basin X is a forested mountain valley with deep soil. Basin Y has had most of its trees cut down, and a large town has been built beside the river on the floodplain.
(a) Identify one natural cause of river flooding. [1 mark]
(b) Explain why deforestation can increase the risk of flooding. [3 marks]
(c) Describe one positive impact of a flood. [2 marks]
(d) Discuss the social and economic impacts of flooding in Basin Y, and suggest how they might differ if it were in an LIC rather than an HIC. [6 marks]
Model answer
(a) Prolonged rainfall (or snowmelt, saturated ground, steep slopes). (1)
(b) Fewer trees means less interception of rain (1). Less water is taken up and given off by transpiration, so soil gets wetter (1). More water runs over the surface to the river faster, so the river rises (1).
(c) The flood leaves fresh silt on the land (1), which makes the soil more fertile for farming (1). (Or: groundwater is recharged.)
(d) Social impacts include people being drowned or injured (1), homes flooded so people are homeless (1), and disease from dirty water (1). Economic impacts include damage to homes, shops and roads in the town (1), businesses closing and jobs lost (1), and large repair costs (1). In an LIC, more people may die or catch disease because there are fewer warnings and slower rescue, and recovery takes longer because there is less money (1). In an HIC the economic cost may be higher because buildings are worth more, though insurance helps people recover (1). A conclusion that compares the two earns the top mark.
Exam tip
In an extended answer, use the three headings (social, economic, environmental) and then add a comparison between LICs and HICs. It shows range, which is what top-level answers have.