After floods: restoring ecosystems.

• Author: Sparks, Richard E.
• Position: Special Section: America Under Water - Cover Story

Decaying riverfronts have been turned into public parks and tourist attractions, and farmland has been revitalized by nutrient deposits.

The flood of 1993 was the result of an unwitting alignment between the forces of nature and the civil works of humans. While it was an economic disaster, it was a boon to many kinds of native plants and animals that not only are adapted to flooding, but depend upon it. Restoration of the natural services of floodplains - including production of fish and wildlife and conveyance and storage of floods - should be evaluated as an alternative to rebuilding agricultural levees, especially at a time when farmers are paid not to grow crops so as to avoid overproduction and costly surpluses.

The flood of 1993 really began in the fall of 1992, when heavy rains left soils saturated throughout the winter, so they could not absorb more moisture when the normal spring rains began in June. Once the rains began, they did not quit. Instead, unusual weather patterns kept pumping warm, moist Gulf air over the Midwest, where the moisture was wrung out by cooler than normal air masses from Canada. Not until the first week in August, 1993, did this pattern begin to break up. By then, portions of the lower Missouri and lower Illinois rivers, as well as much of the Upper Mississippi River, had experienced three flood crests, including the highest one in late July, which reached 49.58' on the St. Louis gauge, uncomfortably close to the 52' design capacity of the floodwall. One more smaller crest arrived in late September before the water finally dropped to the low flow level. One of the cruelties of this flood was that some people began repairing their homes following one crest, only to be hit by another. Not everyone fared as well as St. Louis. Neighboring St. Charles, Mo., for example, almost completely was inundated. Of the 1,576 levees in the Upper Mississippi River Basin, 1,082 were breached or overtopped.

Was the flood an act of God? Certainly in terms of rainfall, this was a very unusual event. Eight of 10 selected National Weather Service stations throughout the upper Midwest received more than 200% of the average July rainfall for the 30-year period 1961-90; three received more than 400%. Should this amount of rain have caused an estimated 12,000,000,000 in flood damage and have any lessons been learned that can be applied in preparation for future floods?

The extraordinary amounts of rain could be expected to produce unusually high river levels, and indeed, peak flood stages at 64 of 154 river gauging stations in the Upper Mississippi River Basin exceeded previously recorded levels. What is both surprising and disturbing is that, at 28 of the 64 stations, the record stages were caused by flows that were below previous records. How could record stages be produced by below-record flows?

River constriction. A look at the historical record of river stages and flows at gauging stations near St. Louis is instructive. Although the 1993 flow was about 20% less than the estimated record flow of 1844, the 1993 crest was 20% greater (about eight feet). There is some uncertainty about the 1844 flow because measurement techniques have changed, but the trend of increasing flood heights for a given river flow is documented by more recent measurements as well. For instance, the 1903 peak flow was just slightly less than 1993's (by about one percent), yet the latter's crest was 11.47' (30%) greater. The 1973 flood stage at St. Louis has been downgraded from a 100-year or even a 200-year event to only a 30-year one, meaning that great floods now occur with increasing frequency. Analyses of the 1973 flood by hydrologist Charles Belt of St. Louis University and a team of river engineers from Colorado State University indicated a significant man-made contribution, caused by constriction of the main river channels by wing dams...