The Great Mississippi River Flood of 1993: Causes and Impact

Overview and Scale of the 1993 Flood

The Great Mississippi River Flood of 1993, also known as the Great Flood of 1993, is considered unparalleled in magnitude, extent, and impact, and is possibly the costliest and most devastating natural disaster to affect the United States (National Oceanic and Atmospheric Administration, 1996). Before it subsided in October of that year, it had inundated 20 million acres across nine states, damaged or destroyed roughly 50,000 homes, and forced approximately 54,000 people to evacuate. Total damage costs were estimated at $20 billion. The flood severely disrupted transportation for almost two months. The U.S. Army Corps of Engineers reported that 40 out of 229 federal levees and 1,043 out of 1,347 non-federal levees were overtopped or damaged (NOAA).

This record flooding occurred from May to September 1993 across North Dakota, South Dakota, Nebraska, Kansas, Minnesota, Iowa, Missouri, Wisconsin, and Illinois (Larson 1996). Its magnitude and severity were so overwhelming that the flood ranks as one of the greatest natural disasters ever to strike the United States. Around 600 river forecast points in the Midwest exceeded flood stage simultaneously, and the event affected almost 150 major rivers and tributaries. Fifty flood-related deaths were reported (Larson).

Meteorological Causes and Hydrological Conditions

The causes of the Great Flood were attributed to several compounding factors: soil saturation and elevated stream levels in the fall of 1992, snowpack across the central United States, heavy rainfall in late March, saturation of the northern reaches of the Missouri River basin, and an unusually persistent weather pattern from June through early August (Larson 1996). The Bermuda High pressure system was stronger and positioned farther north and west than usual, which prevented storm systems from following their normal track across the Ohio Valley. Instead, storms regenerated repeatedly over the Central Plains, depositing large amounts of rainfall across the nine affected states. Some areas received more than four feet of rain.

Record flooding was recorded at approximately 500 weather forecast points. Many locations remained above flood stage for three to five consecutive months, with the Mississippi River staying above the 32-foot flood stage for 126 days. Floodwaters from the River spilled into the Big Muddy and Ohio Rivers, causing Murphysboro and Cairo to exceed flood stage as well. Damage was traced along the Mississippi River on both the Missouri and Illinois sides. In Cape Girardeau, Scott, Mississippi, and New Madrid Counties, close to 100,000 acres of rich farmland were lost to the flood at a cost of approximately $22 million. Around 2,400 people were evacuated from Jackson, Union, and Alexander Counties (NOAA). At least 10,000 homes were totally destroyed, and hundreds of towns were damaged or completely submerged (Larson 1996).

A chief cause of the record flooding was the nearly continuous rainfall. Many places in the nine affected states recorded as many as 20 or more rainy days per month, compared with the average of only 8–9 rainy days. This persistent rain-producing weather pattern across the Upper Midwest enhanced the daily development of rainfall throughout much of that summer. Heavy rainfall in June and July, combined with already saturated soil conditions, caused severe flooding. In early July, widespread rains fell across Iowa, producing record flooding on the Skunk, Iowa, and Des Moines Rivers. Des Moines was particularly hard-hit on July 9. Heavy rains then shifted westward in late July toward North Dakota, Nebraska, Kansas, and Missouri. There were 92 new locations with record flood crests during the event.

The flood was also unusual in other ways. It was geographically widespread, covering nine states and approximately 400,000 square miles; it topped or damaged more than 1,000 levees; and it was of extraordinary duration, lasting nearly 200 days in some locations. As researchers noted, cohesive soils minimized the flood's erosional impact, but the contrasting geomorphological response of different reaches of the Upper Mississippi River floodplain to levee failure clearly illustrated the necessity of accounting for erosional resistance thresholds—in addition to flood hydrology—when assessing the sensitivity of floodplains to rare, high-magnitude events (Gomez et al.).

Environmental and Ecological Impact

The Mississippi River flood took 46 lives and destroyed thousands of homes as it reclaimed approximately 64 million acres of land (Gutin 1994). It caused as much as $10 billion in damage and significantly altered the fate of numerous plant and animal species living in and around the River. Several of the oldest and tallest cottonwood trees—which served as roosts for the endangered bald eagle—were swept away by the fast-moving floodwaters. Herbicides carried in agricultural runoff flowed into the Gulf of Mexico during the five weeks of peak flooding at a rate far exceeding normal annual flows. Biologists expressed concern that this could damage the aquatic vegetation relied upon by migratory waterfowl. Other organisms adversely affected included the paddlefish, the walleye, and the northern pike.

At the same time, the flood may have helped some native species reclaim territory lost to invasive competitors. Ecologists believed that the River would ultimately manage its own ecological recovery (Gutin). The long-term balance between flood-related harm and habitat renewal remained an active area of scientific inquiry in the years that followed.