Physical Setting

The flow of the Mississippi River ranks as the world's third largest behind the Amazon and Congo. The outflow of the river into the Gulf of Mexico averages 173,600 m3/s and represents 5% of all the freshwater discharged into the oceans of the world (Tarbuck and Lütgens, 1984). The variability of precipitation falling in the basin is large, reflected in a 1-year record low flow of 75,000 m3/s, and a record 1-year high of 600,000 m3/s, more than three times the long-term average. The basin occupies all or part of 34 states and 41% of the contiguous United States, sprawling over 3.2 million km2. Headwaters exist in the Rocky Mountains of the west, the Appalachian Mountains of the east, and the forests of the northern United States. (Fig. 1).

The enormous basin embraces five major climatic regions including humid continental, semiarid steppe, and wet subtropical climates. Four very different physiographic regions are found in the basin including the world's most productive soils and seven very different vegetative regions (White et al., 1979). This diversification in the physical setting greatly affects the type and frequency of floods that occur. Millions of years ago, the river's delta began forming near Cairo, Illinois (Fig. 1), and subsequently advanced 1600 km to the south. New Orleans, the river's major port city, rests where ocean waters existed only 5000 years ago. The enormity of the basin's erosion and sediment transport of the river is shown by the fact that the river deposits 750 million tons of sand, silt, and clay annually into the Gulf of Mexico (Tarbuck and Lütgens, 1984). The sediment load, resulting from the record 1993 flood, led to a discharge of 2.1 billion tons (Bhowmik, 1996).

The basin is so large that it embraces two other enormous river basins—those of the Ohio and Missouri Rivers, and the Mississippi alone is so huge that it is divided hydrologically at St. Louis into the upper and lower basins. Due to climatic differences and the basin's enormity, there has never been a flood that encompassed the entire Mississippi basin. The net effect of the basin's physical situation (size and climatic factors creating floods) is that floods only occur at a given time on one, or infrequently two, of these large basins like the Ohio, the Upper Mississippi, or the

Figure 1 Mississippi River System and its main rivers.

Missouri. The areas most prone to large-scale simultaneous floods are the Ohio River and Lower Mississippi.

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