It has been suggested that unusually heavy precipitation beginning in the year leading up to the flood was the result of excess cloud condensation nuclei from the earth-circling ash cloud generated by the massive eruption (the second largest of the 20th century) of the Philippines volcano Mount Pinatubo in 1991. A rainy autumn in 1992 resulted in above normal soil moisture and reservoir levels in the Missouri and Upper Mississippi River basins, and during the winter of 1992-93, the region experienced heavy snowfall. These conditions were followed by persistent spring weather patterns that produced storms over the same locations. These wet weather conditions contrasted sharply with the droughts and heat waves experienced in the southeastern United States.
Storms, persistent and repetitive in nature during the late spring and summer, bombarded the Upper Midwest with voluminous rainfall amounts. Portions of east-central Iowa received as much as 48 inches of rain, between April 1 and August 31, 1993, and many areas across the central-northern plains had 400-750% above normal precipitation. In the St. Louis National Weather Service (NWS) forecast area encompassing eastern Missouri and southwest Illinois, 36 forecast points rose above flood stage, and 20 river stage records were broken. The 1993 flood broke record river levels set during the 1973 Mississippi and the 1951 Missouri River floods.
In April, the Mississippi River had crested 6 to 10 feet (2 to 3 m) above flood stage and once again approached near the same levels during the month of May. In the beginning of June, the rivers dropped below flood stage and were receding. During the second week of June, river levels rose to near flood stage before yet again beginning their slow recession. By the end of June, the Mississippi River was four feet (1.2 m) below flood stage at St. Louis, while many other river locations in the region were near flood stage. Precipitation for the month averaged from one inch (25 mm) above normal at Kansas City, to nearly four inches (100 mm) above normal at Springfield.
The Redwood River in Minnesota began experiencing severe flooding in May.
In June, flooding occurred along the Black River in Wisconsin, with flooding also starting to occur along the Mississippi, Missouri, and Kansas Rivers. Starting as early as June 7, reports of levees being overtopped and levee breaks became common. These breaches acted to delay the flood crests, temporarily storing excess water in the adjacent lowlands, but the rain kept falling.
July brought more heavy rain to the Missouri and upper Mississippi River basins in Missouri, Iowa, Kansas, Nebraska, North and South Dakota, Illinois and Minnesota. Rainfall amounts of 5 to 7 inches (125 to 175 mm) in 24 hours were common. Precipitation for the month averaged from one inch (25 mm) above normal at St. Louis and Springfield, to between six and seven inches (150 to 175 mm) above normal at Columbia and Kansas City, Missouri. The copious rain amounts during July sent record setting crests down the Mississippi and Missouri Rivers, causing river gauges to malfunction along the way. Both record crests met each other at their confluence near St. Louis within days of each other.
From July 11 until July 22 the Des Moines, Iowa Water Works treatment facility was flooded by the Raccoon River. This resulted in the plant being powered down and no running water for that period. During this time the Army National Guard and American Red Cross set up water stations, and the local Anheuser-Busch bottler distributed water in white 6-packs with their logo on it. Once running water was restored there was enough pressure to bathe and flush toilets, but the water was not certified potable until July 29. The final usage restrictions were lifted in August.
The Mississippi River stalled a few days at the April 1973 record stages, seemingly waiting for the Missouri River to arrive, before pushing levels upwards again, breaking levees, driving people and their possessions to higher ground and causing havoc with anything in its path.
The crests, now combined as one, moved downstream through St. Louis and Chester on its way to the confluence with the Ohio River at Cairo, Illinois. Only minor flooding occurred below the Ohio due to the river's larger channel below that point, and the drought the eastern U.S. was facing at the same time; if the Ohio River watershed had not been in drought conditions, the 1993 flood might have rivaled the 1927 flood in overall damage on the lower Mississippi.
Major sandbagging activities were underway on the lower Missouri River, the River des Peres in St. Louis, the Mississippi River south of St. Louis and on many other tributaries across Missouri and Illinois. Some of these efforts were successful while others were not as the river continued to spawn destruction.
Over 1,000 flood warnings and statements, five times the normal, were issued to notify the public and need-to-know officials of river levels. In places like St. Louis, river levels were nearly 20 feet (6 m) above flood stage and had never been this high in its 150 year history. The 52 foot (16 m) St. Louis Floodwall, built to handle the volume of the 1844 flood, was able to keep the 1993 flood out with just over two feet (0.6 m) to spare.
On August 1, levee breaks near Columbia, Illinois flooded 47,000 acres (190 km²) of land, inundating the towns of Valmeyer and Fults, Illinois. The released water continued to flow parallel to the river, approaching the levees protecting historic Prairie du Rocher and Fort de Chartres. On August 3, the Corps of Engineers made the decision to break through the stronger Mississippi River levee to allow the water back into the river. The plan worked and the historic areas were saved.
On the Missouri River it was estimated that nearly all of the 700 privately built agricultural levees were overtopped or destroyed. Navigation on the Mississippi and Missouri River had been closed since early July resulting in a loss of $2 million (1993) per day in commerce.
The Mississippi River at St. Louis crested at 49.6 feet (15.1 m) on August 1, nearly 20 feet (6 m) above flood stage and had a peak flow rate of 1,080,000 ft³/s (30,600 m³/s). At this rate, a bowl the size of Busch Memorial Stadium would be filled to the brim in 69 seconds.
Costs and damage
Some locations on the Mississippi River flooded for almost 200 days while locations on the Missouri neared 100 days of flooding. On the Mississippi, Grafton, Illinois, recorded flooding for 195 days, Clarksville, Missouri, for 187 days, Winfield, Missouri, for 183 days, Hannibal, Missouri, for 174 days, and Quincy, Illinois, for 152 days. The Missouri River was above flood stage for 62 days in Jefferson City, Missouri, 77 days at Hermann, Missouri; and for 94 days at St. Charles in the St. Louis metropolitan area. On October 7, 103 days after it began, the Mississippi River at St. Louis finally dropped below flood stage. Approximately 10,000 homes were destroyed as a result of the flooding, with 15 million acres of farmland inundated, and the whole towns of Valmeyer, Illinois and Rhineland, Missouri were relocated to higher ground. The floods cost twenty eight lives officially; however, a more likely target is suspected to be around fifty people, as well as an estimated 15-20 billion dollars in damages.
Comparison to other big floods
Channeling and levee construction have altered how the floods have hit various areas along the Missouri River. Here's a comparison of the three big floods since the early 1800s.
Great Flood of 1844 - This was the biggest flood of the three in terms of rate of discharge at Westport Landing in Kansas City. It is estimated that 625,000 cubic feet per second (17,700 m³/s) was discharged in the flood. However the crest on July 16, 1844, was almost a foot (0.3 m) lower than the 1993 flood.
Great Flood of 1951 - The 1951 flood was the second biggest in terms of rate of discharge at 573,000 ft³/s (16,200 m³/s). The 1951 crest on July 14, 1951, was almost two feet (0.6 m) lower than the 1844 flood and three feet (1 m) lower than 1993. However, the flood was the most devastating of all modern floods for Kansas City since its levee system was not built to withstand it. It destroyed the Kansas City Stockyards and caused Kansas City to build Kansas City International Airport away from the Missouri River bottoms to replace the heavily damaged Fairfax Airport in Kansas City, Kansas.
Great Flood of 1993 - The 1993 flood was the highest of any of the three but had the lowest discharge at 541,000 ft³/s (15,300 m³/s). While the 1993 flood had devastating impacts elsewhere, Kansas City survived it relatively well because of levee improvements after the 1951 flood.