Water overflows（溢出）onto dry land and a flood happens.Floods have always been a part of life on earth.Almost every culture has a tale about a great flood.Since old times，people have built the cities along rivers because they use the water for drinking and for farming.For that reason，river floods affect（影响） many people.In some places people have built dams（堤坝） to protect lands from river flooding.
    Floods can be dangerous.However，floods are not always bad.When muddy floodwaters go down，they sometimes leave a layer（层） of wet earth.People in ancient Egypt depended on the flooding of the Nile River every year to help them grow their crops，like rice and corn.
    Most often flooding is caused by heavy rains over a long time.After a lot of rain，the ground becomes full of water，and new rainwater runs off，or flows downhill.That runoff goes directly in streams and rivers.In springtime，the rain may be mixed with melting（融化的） snow.Because of the extra（多余的） water flowing into the streams and rivers，their water level（水位线） rises.If the level rises so much that the water overflows the banks，a flood happens.
    Flooding can be worse if there is not much vegetation （trees，grass，and other plants） on the land around rivers.In general，land that has little vegetation is less able to absorb（吸收） water.The result is a larger amount of water entering a river system.
    Flash floods are the deadliest kind of flood.They are caused by sudden heavy rainfall or a broken dam.They happen very quickly and can catch people before they have a chance to get away.Flash floods often happen in hilly country or around dry riverbeds.In a flash flood，a completely dry riverbed can fill in minutes.

About floods
How floods happen	Floods happen（1） with with water overflows. A long time of heavy rains makes rainwater runoff enter streams and rivers，and their serious water level rises bring water overflows.
The（2） good side good side of floods	The flooding（3） provides provides people with with a layer of wet earth to help them plant crops.
Why floods can be worse	Land with few trees and grass（4） is short of is short of the ability to absorb water.
The most terrible kind—flash floods	Place：hilly countries and dry riverbeds. （5） Causes Causes ：sudden heavy rainfall or a broken dam. Results：deaths and the quick filling of the riverbed.