"When the Mississippi River reaches 87.9 m above sea level near New Madrid, Missouri, bottomlands adjacent to the river and
farmland, roads, ditches and wetlands begin to flood. Concurrently, at the lower end of the New Madrid Floodway the rising Mississippi backs up into Main Ditch (figure 1), the 454 m (1,500 ft) gap in the frontline levee designed to drain the Floodway and St. Johns Levee and
Drainage District to the river. When this occurs, the Main Ditch gates on the setback levee (figure 3) are closed to protect the St. Johns Bayou basin from Mississippi River backflow. However, with the Main Ditch gates closed, precipitation within the basin has no outlet as it drains to the Main Ditch channel. This causes tributary streams to back up and flood a substantive portion of agricultural lands and the town of East Prairie, Missouri."
View the recent article; "St. Johns Levee and Drainage District attempt to mitigate internal flooding" published in the Journal of Soil and Water Conservation, by Dr. Kenneth R. Olson, Professor Emeritus, NRES, and Dr. Lois Wright Morton of the University of Iowa: http://www.jswconline.org/content/71/4/91A.full.pdf.