A Regional Perspective of the Hydrology of the 1993 Mississippi River Basin Floods
The Great Flood of 1993 was one of the costliest natural disasters in American history. This flood was primarily the result of a persistent weather pattern that delivered precipitation across a very large part of the Midwest for an extended period of the summer. In contrast to normal years when most places in the region receive about half of their annual precipitation between May and August, many places received the equivalent of the annual rainfall in that time period in 1993. Much of this rain fell on soils that were already saturated and unable to store additional runoff. Annual and monthly rainfall totals for the states of Iowa, Minnesota, and Illinois for 1993 were the highest in 100 years. Precipitation was highest in Iowa, where the annual total precipitation of 1,200 mm was estimated to have an exceedence probability of 0.001. Peak discharges on the upper Mississippi and lower Missouri Rivers were likewise characterized by low probabilities, but these estimates can be interpreted very differently depending on the assumptions used in the analysis. In contrast to the main-stem rivers, flooding on tributary streams, whether expressed in terms of return period or discharge per unit drainage area, was less extreme. Arguments put forth in the popular press and elsewhere that land-use modifications in the Mississippi River basin exacerbated flooding in 1993 are reviewed in detail, and it is suggested that these effects may be important for floods with return periods < 50 years but have much less influence on floods of this magnitude.