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Floods

 



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Overview
It's hard to believe that a small, slow-running stream could cause harm. During a flood, however, even the calmest brook can become a torrent and cause unforeseen misery and damage. Many people like to live near a body of running fresh water. Rivers and streams provide recreation, food, and, of course, drinking water. But when rain or melting snow produces large amounts of water, rivers and streams may flood, causing problems for those who live nearby. Although we describe floods as disasters, they are, in fact, natural phenomena. Floods occur when a watershed receives so much water that its waterways cannot drain it off properly. A watershed is an area of land (usually quite large) that drains into a river or stream. A small river will drain several thousand or hundreds of thousands of acres of land. Within any one watershed, excess rain will cause increased water levels downstream. What occurs at any point along a river can affect not only that point but also the entire watershed. To minimize the effects of a flood, engineers build levees to constrict the overflow of rivers. As more communities build levees, the water in a river is forced to run at a higher level because it cannot spread out. As the water runs at a higher level, it deposits sediment and raises the riverbed. The situation worsens as the water rushes downstream. The water level can only continue to rise, eventually spilling over the levees. During prolonged periods of flooding, many levees give way because they are under pressure from the swollen river and are being undercut by water seepage. Floods in undeveloped areas are not as damaging as the floods in developed areas. First of all, many natural areas have thousands of acres of wetlands which act as giant sponges to soak up excess water. Second, many rivers overflow into the floodplain --a low, flat area on either side of the river. If a river is allowed to spread out onto its floodplain, the flow downstream is slowed. A river's floodplain can accommodate huge amounts of water which are diverted from the main channel and held back. Allowed to flood in this way, the river creates less damage downstream. If humans do not interfere with it, a stream or river produces its own flood control system.

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