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USGS CMG InfoBank: Levees

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Comment: 13:14 - 14:42 (01:28)
Source: Annenberg/CPB Resources - Earth Revealed - 19. Running Water I: Rivers, Erosion and Deposition
Keywords: "Douglas D. Rhodes", river, floodplain, sediment, deposition, flood, "river channel", stream, discharge, "river bed", levee

Our transcription: The edges of flood plains are marked by "levees", ridges of sediment left atop riverbanks by floods.
Once formed, levees serve as natural barriers confining rivers during periods of ordinary flow.
They may even protect low-lying areas from flooding if the level of a river isn't too high.
For this reason, artificial levees designed to contain a river during flood stages are often built.
But artificial levees can themselves create problems.
By confining the river to a narrow channel, levees may also confine sediment, raising the river bed higher and higher.
And levees can provide a false sense of security.
If a river overtops its levees to flood the surrounding land, the levees can actually prolong flooding by preventing water from draining back into the river.
Most people don't appreciate the fact the floodplain is very much a part of the stream itself, not something separate from it.
The floodplain is the place where rivers store discharge during periods of high flows, and also places where rivers store sediment during periods of low flows.
When we move on to the flood plain, we're moving on to the river, and it really isn't very much different from being in the channel itself.
It's just that the river doesn't use it all of the time.

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