Causes of the Mississippi Flood in 1993

Typical Activity

• Flooding in the Mississippi area is a normal occurrence in spring. Flooding occurs here when rain and melted snow enter the water system and end up in the Mississippi Basin. However, this is normally followed by a spell of dry weather and drought, which is also a normal feature of this land. In 1993, the dry period did not arrive as usual because of the atypical weather systems of that year.

Saturated Soil

• The earliest causes of the Mississippi flood in 1993 began to occur almost a year before the event, in the fall of 1992. Because 1992 was not as warm as normal, less water evaporated from ground level. This meant that rainfall ran into the rivers and streams as it could not be absorbed by soil that was already saturated. Further rain in the winter months exacerbated the problem, followed by more rain and melted snow in the spring of 1993. Far more water than normal had to run into streams and rivers.

Unusual Atmospheric Conditions

• Leading up to the flood in 1993, abnormal patterns of atmospheric conditions were experienced in North America. A persistent jet stream of warm, moist air ran from the Gulf of Mexico to the Midwest. Eventually, this jet stream met with dry, cold air traveling south from Canada, which had been experiencing colder weather than normal. The resulting weather system featured a multitude of thunderstorms and heavy rainfall from June through August, with the two high pressure systems preventing the storms and precipitation from moving to another location. The heavy rainfall continued to fall onto already saturated soil and into the full Mississippi River.

The Human Contribution

• Human intervention in the Mississippi River system through engineering and urbanization also contributed to the extent of the flood in 1993. A large proportion of the wetlands surrounding the Mississippi River have been drained by humans in order to use the land for agricultural purposes and through urbanization. To replace the natural role that these wetlands had previously played in absorbing flood waters, systems of flood walls and levees had been engineered to increase the amount of water that could be contained within the bounds of the Mississippi River. The result of this is that a powerful flood with the ability to overcome these human-engineered systems, such as that of 1993, will spread to an even greater area than that which was covered by the original floodplains.