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The Annual Flows of the Mississippi River

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The Mississippi River drains two thirds of the lower United States plus 13,000 square miles of Canada. When North America was being colonized by Europeans, the river overflowed its banks about once every 3 years and spread onto the floodplain, which today covers 34,600 square miles of the valley. A natural levee formed alongside the river where the silt was dropped when water left the channel; the levee now slopes away from the river at about 7 feet per mile. This high ground was settled first by the white man at New Orleans in 1717. The spring floods barely topped the natural levee and the original town was protected by a ring levee 3 feet high. As more overflow areas were cut off from the river, the levees increased in height to about 40 feet. The hydraulics of the river became better and today more water and silt flows out to sea. About three fourths of the floodplain is closed off from the river, but in 1882 and 1927, the river took that land back, and in 1973 almost 60% of the 22-million-acre area was flooded. Nevertheless, there have been no levee breaks since the Corps of Engineers took over flood control in 1928.

The mean flow of the river since 1900 has been 646,000 cubic feet per second (cfs) moment to moment. The mode, median, quartiles and deciles of annual flows are given, and the measurements of dispersion, the standard deviation and coefficient of variation are given.

The Atchafalaya River distributary has increased considerably at the expense of the Mississippi River since 1858. During the flood year of 1973, the Atchafalaya carried 37% of the total flow. It is estimated that unless it is brought under control, in about 60 years the Atchafalaya will equal the Mississippi.

Flood years are not especially associated and in several cases low flows and flood years are close together.

Measurements of river flows before 1900 are unreliable or absent. Since then, however, careful measurements of the daily flows of both distributaries have been taken by the Corps of Engineers and used to compile mean flows in cfs by years. The data extend for a series of 79 years. They were furnished to the author by the New Orleans District of the Corps. These data were used for all calculations given here on flows. The lowest flow recorded for the Atchafalaya was 13,300 cfs on September 22, 1925. The lowest flow for the Mississippi was 75,000 cfs on November 4, 1939. The highest for the Atchafalaya was 781,000 cfs at Simmesport on May 12, 1973; the highest for the Mississippi was at Tarbert Landing on February 19, 1937, at 1,977,000 cfs. Subjectively described floods of 1782,1828, and 1882 tie in with 1927 and 1973 as 50-year floods. The 1927 and 1973 floods were remarkably similar; the former was the larger. The largest known flow of the river is only 25% less than the maximum which meteorologists say could be generated. Presumably such a flood could be handled without catastrophe.

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