River flooding occurs when heavy rain or snowmelt results in more water than the river can hold within its banks. When rain falls on saturated or impermeable surfaces like rock, soil, or paved areas, the water is unable to soak into the ground and instead flows quickly overland into rivers. This excess surface runoff causes rivers to overflow their banks and flood surrounding areas. Factors that increase flooding risk include soil saturation, soil and rock impermeability, steep slopes, deforestation, and urban development which replaces permeable land with impermeable surfaces.

1. River flooding

2. Why is flooding a problem?
• Because it is an increasing problem across the world.
• Even if flooding has not affected you yet, it could easily do some time in
the future.
• For this reason, we need to understand the causes and affects of
flooding so that we can try to manage the problems they create.

3. Look at the people in the photos.
• What problems do they face?
• What help do they need?
• Who needs the most help?
• How could they prepare for the situation?
• What could you do to help them?

4. What causes a river to flood
• All the water that flows down a river comes from rain or melting snow.
Sometimes after heavy rain or rapid snow melt, there may be too much
water for the river to hold.
• The river will then overflow its banks and spread out across the land on
either side of its channel. This is called a river flood.
• Usually, when it rains, most water simply soaks into the ground and there is
little chance of a flood.
• If, however, the water is unable to soak into the ground, it will stay on the
surface and flow quickly downhill and into the river. This is when floods are
most common.

5. • Some rivers are more at risk from flooding than others.
• Put simply, heavy rain and anything which stops that rain from soaking into the
ground will increase the chances of flooding.
• Some of the factors that increase the risk of flooding are shown below.
(i) Rock and soil type – Impermeable rocks and soils do not allow rain to soak
through them. Any rain that falls will stay near the surface.
(ii) Very wet soil – If rain has been falling for sometimes, the soil may become full
of water. Any further rain is unable to soak into the ground and remains on
the surface.
(iii) Very dry soil – Soil that is baked hard by the sun in dry weather builds up a
crust. Rain is unable to soak through the crust and so remain on the surface.

6. (i) Steep slopes- Rain falling on the steep slopes runs quickly downhill
towards a river. It has little time to soak into the ground, so most stays
on the surface.

7. • There are two ways in which humans may increase the risk of flooding
are by cutting down trees and building more towns and cities.

8. Now solve this out… (pg-109, no.2)
• Human activity (H) or Natural
causes (N)
Cutting down trees increases the
risk of flooding.
If soil is full of water, no more rain
can soak into it.
Impermeable rocks and soils do not
allow water to soak in.
Water flows across concrete and
tarmac quickly to the river.
Water flows quickly down steep
slopes. Little soaks in.
Gutters and drains carry water
directly to the river.