The Mississippi River is the sixth largest river in the world, pouring an average of 173,600 cubic meters (227,000 cubic yards) of water per second into the Gulf of Mexico. As with most rivers, the rate of the Mississippi's flow varies seasonally. Generally, it carries far more water between March and May than it does in late summer. In July of 1993, however, the upper parts of the river and its tributaries reached levels far in excess of those anticipated by the engineers who designed the levees built to contain the rivers. As a result, the river system flooded communities in nine Midwestern states and left more than 50,000 homes damaged or destroyed.

The Great Flood of 1993, as it is now called, ranks as one of the worst natural disasters and one of the costliest floods in United States history. Even so, some have called it a leisurely disaster because it resulted not from a single storm, but from an anomalous weather pattern that took shape and remained in place for months during the summer of 1993.

Most thunderstorms form near surface frontal features, such as a cold front, that are associated with the polar jet stream. This high-speed band of wind travels in the atmosphere about 10 to 15 kilometers (6 to 9 miles) above Earth. It typically occurs above the boundary between warm, tropical air masses to the south and cooler air masses to the north. In the summer, the air mass on the tropical side may be moist and unstable, thus triggering thunderstorms. These thunderstorms may develop into long-lived clusters, or convective systems, near the jet stream. Thus, the position of the jet stream is a good predictor of where convective systems will form and to where they will travel.

Typically, the jet stream's course undulates, changing position over a period of days or weeks. For much of the summer of 1993, however, the jet stream remained almost stationary over the Great Plains and upper Midwest. A high-pressure system to the south provided a constant source of warm, moist air from the Gulf of Mexico, and evaporation from the saturated soil surface provided even more moisture to the already unstable air mass. When this moist, unstable air mass intersected with the cooler, drier air mass to the north, thunderstorms occurred. In some areas, this endless parade of thunderstorms dumped nearly 100 centimeters (more than 3 feet) of rain during the month of July. This heavy precipitation fell onto soil already saturated from a typically wet spring and had nowhere to go but into the rising waters of the Mississippi River basin.

Construct a model of a river system with levees in this NOVA classroom activity.

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