The causes of flooding

Weather

Rainfall (volume) 

A lot of rain causes higher river levels, whereas less rainfall or prolonged dry periods will result in lower river levels due to both the volume of rain and the fact that it will take longer for the rainwater to reach the river. 

Rainfall (type)

The type of rainfall can also have an influence as long periods of heavy rain can saturate the soil and underlying rock, meaning further rainfall will flow as surface run off towards the river. Light rain or drizzle has time to infiltrate the soil freeing up space for more rain. 


Snow

Melting snow can cause large volumes of water to saturate the land and run off towards the river. 

Physical Factors

Relief

Steep slopes encourage fast run off as the water spills rapidly downwards due to gravity. On gentle slopes, absorption into the land reduces the volume of water reaching the river. 


Land use 


Urbanisation (including the building of roads, houses and hard standing)

Creates an impermeable surface preventing rainwater soaking into the ground. Man made drains are very effective ways of transporting water away from the built up areas, and into rivers. 

Deforestation

Allows water to reach the surface quicker as the trees do not intercept it and the trees also do not extract water from the ground.