In the summer of 1993 the United States were faced with the most devastating flood that has ever occurred. Seventeen thousand square miles of land were covered by floodwaters in a region covering all or parts of nine states (North and South Dakota, Nebraska, Kansas, Missouri, Iowa, Wisconsin, Minnesota and Illinois). All large Midwestern streams flooded including the Mississippi, Missouri, and Kansas, Illinois, Des Moines and Wisconsin rivers. The Mississippi river was above flood stage for 144 days between April and September and approximately 3 billion cubic meters of water overflowed from the river channel onto the floodplain downstream from St. Louis. There were 4 principal reasons why flooding was so extensive:The region received higher than normal precipitation during the first half of 1993. Much of the area received 150% of normal rainfall and some states received more than double their average rainfall.Individual storms dumped large volumes of precipitation that could not be accommodated by local streams.The ground was saturated due to cooler than normal conditions during the previous year which means less evaporation so rainfall was absorbed by soils and more ran-off into streams.The draining of riverine wetlands and the construction of levees had altered the river system over the previous century.The Mississippi river is divided into two parts. The Upper Mississippi runs from its source to Thebes, southern Illinois, where the Ohio River meets the Mississippi. The Lower Mississippi runs downstream from Thebes to the Gulf of Mexico. Flooding was confined to the Upper Mississippi because the river channel widens considerably south of Thebes, and the Lower Mississippi received lower than average inflow from tributaries. Discharge increases steadily as more tributaries add water to the river.Some of the environmental damage the flood contributed to include: a lower oxygen concentration (below 2mg/L) was elevated and more widespre...
