Flooding: who is to blame?

• Author: Tripp, James T.B.
• Position: Special Section: America Under Water - Cover Story

By building on historic floodplains, man is as responsible as nature for the devastation of rampaging rivers.

During the summer of 1993, the Upper Mississippi River Basin and Lower Missouri River Basin experienced levels of flooding unprecedented in recent history. The entire floodplains of vast stretches of these rivers and their tributaries were filled with water. The impact on agricultural and urban property owners was devastating. People had forgotten that these floodplains' soils and topography have been shaped by periodic floods over thousands of years. The question for the affected communities and the nation's taxpayers who had to finance much of the cleanup, rebuilding, and disaster relief is whether it is time to come to a new understanding about the most appropriate use of the floodplains of the Upper and Lower Mississippi River Basins. To do this, it is necessary to understand what a floodplain is.

The Mississippi is one of the six largest rivers in the world in terms of the volume of water and sediment it transports to the sea. As Mark Twain pointed out in Life on the Mississippi, if the Missouri River is considered the headwaters of the Mississippi, the later would be the longest river in the world. It is an awesome experience to witness the confluence of three rivers - two originating in Yellowstone National Park, at Three Forks, Mont., to form the Missouri River and the joining of the Allegheny and Monongahela Rivers that form of the Ohio River at Pittsburgh - and realize that all of these waters are flowing to the Mississippi River and the Gulf of Mexico.

The floodplains of the Mississippi River and its tributaries are enormous. Those of the Lower Mississippi River below St. Louis alone occupy some 25,000,000 acres. Historically, the lowest-lying portions of these floodplains - from Minnesota to Louisiana - typically were flooded every year as river waters rose in response to late winter and spring snow melt and rains. During periods of more ran and snow melt over a larger area, the affected rivers would rise and fill larger portions of the floodplains. During very high water occasions. these floodplains would be filled with water. Moreover, when the Mississippi River water levels are high, tributaries of the river in the Upper and Lower Basin can not discharge efficiently. Their waters back up, flowing into their floodplains. In the Lower Mississippi River Basin, these tributary floodplains are known as backwater or overflow areas, huge floodplains that hold and store waters that can not discharge until Mississippi River flood stages go down.

Before the vast changes wrong by European settlers, the floodplains of the Mississippi River and its tributaries extensively were covered by hardwood forests. They would be classified as wetland forests since the species of plants found in the floodplains can survive and grow under prolonged flood conditions. Upland forest plant species usually were not found in these floodplains because they could not survive flooding conditions.

The Mississipi is a great alluvial river. It carries large amounts of sediments, some 200,000,000 tons of sediment per year on average to the Gulf of Mexico, where, over thousands of years. the river has build one of the greatest deltaic systems in the world. The Mississippi deltaic plain in Louisiana contains almost 4,000,000 acres of saline, brackish, and freshwater wetlands. Historically, when the river flooded, its sediment-ladened...