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COPYRIGHT 1994 American Geographical Society
In 1539 members of Hernando DeSoto's expedition witnessed extensive flooding along the lower Mississippi River. This was the first of many floods to be recorded by Europeans, who also took note of how the indigenous inhabitants coped with such flooding. The Indians, they wrote, "built their houses on the high land, and where there is none, they raise mounds by hand and here they take refuge from the great flood." As Europeans settled - and cleared - the Mississippi River basin, it is unfortunate that they did not follow the example set by the Indians. Indeed, each time the Mississippi River flowed out of its banks onto its flood plain, the lesson had to be relearned: the Mississippi's use of its flood plain would not be denied. Nor would the flood-plain settlers or the upland farmers learn the principle of watershed unity that actions upstream could have an impact on areas far downstream. Thus in the uplands, forests were cut, prairies plowed and wetlands drained as tens of thousands of westward moving settlers converted the land to agricultural production. Unknowingly, they began the process which ultimately led to the Great Flood of 1993 as well as earlier floods: as the land was cleared, precipitation ran off the land into the rivers faster and in greater quantity than before - the natural vegetation was no longer there to slow and retard runoff. Thus there were not only more frequent floods but floods of greater magnitude.
The flood plain's form and function
A river's flood plain is central to any discussion of floods, as shown in the accompanying diagram. A broad, nearly flat landform consisting of stream-deposited sediment, the flood plain is inundated by flows in excess of channel capacity (that is, a flood). The flood plain is higher along the edge of the river, built up by deposition of sediment from water overflowing stream banks. As the water leaves the channel, it is abruptly slowed by the reduced gradient and friction of the flood plain. The deposition of sediment along the channel banks produces natural levees which appear as slightly higher ground between the river and its flood plain. The natural levee slopes, often imperceptibly, away from the river so that the lower and less well-drained portion of the flood plain often has standing water, known as a backswamp.
The flood plain, including the back-swamp, is extremely important in the natural process of flooding. If a flood is defined as flow in excess of channel capacity, then the flood plain's role is to store the excess flow until it can be accommodated by the channel. Although some flood water evaporates, most of it flows directly back into the channel or rejoins the channel indirectly by groundwater flow. The flood plain is instrumental in flood control by storing - and slowing - excess water.
In 1993 the Mississippi River and its flood plain differed considerably from what members of DeSoto's expedition saw in 1539. Then the Mississippi meandered freely across its flood plain, uninhibited by levees and dams. The flood plain was largely forested and wetlands were common as were oxbow lakes created by shifting channels. Flood-plain ecosystems, especially wetlands, were home to an especially rich and diverse assemblage of plants and animals. Rich alluvial sediment and an...
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