What causes floods?
Flooding occurs when water cannot drain away fast enough in the rivers. In areas of non-porous rock, water runs off the land very quickly and streams and rivers soon overflow. Flooding also happens when winter snows thaw in spring. Huge floods cover parts of Siberia every spring, when snow melts while the rivers are still iced up. Low-lying coastal lands are vulnerable to flooding, especially when gales and high tides cause water to flow inland. Low-lying Bangladesh is particularly liable to this kind of flooding. In addition, melting snow in the Himalayan Mountains adds huge amounts of water to Bangladesh’s rivers, increasing the flood risk.
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Where were the worst floods?
Some of the most terrible floods known were in China along the Hwang-he, or Yellow River. The river burst its banks in 1931, killing more than 3.5 million people. For centuries, dykes have been built to prevent flooding, but these burst when the water flow is too great. Huge dams and sluice gates have been installed to reduce the risk of flooding.
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