Even before the latest storms, crews had long labored to strengthen the earthen levee at Winfield from dirt slides and spots where water soaks up through the sandy soil.
Lincoln County authorities on Wednesday deemed the area extremely hazardous and ordered boaters out of the water, saying even slight wakes lapping against the levee could cause a catastrophic failure.
The 2½-mile-long levee about 45 miles northwest of St. Louis is all that's protecting 100 houses, a city park, several businesses and 3,000 acres of agricultural land in east Winfield.
Forecasts showed the Mississippi will crest at Winfield at 37.5 feet on Friday, more than 11 feet above flood stage. But thunderstorms are forecast over the coming days upstream in Iowa, Minnesota and Illinois.
At Clarksville, northwest of St. Louis, a weakened sandbag wall protecting the city's historic downtown was reinforced and was holding ahead of Friday's predicted crest of 36.9 feet.
Downstream in Grafton, Ill., the crest forecast for Saturday evening was showing "a foot rise over what they've had so far," hydrologist and meteorologist Mark Fuchs said. A crest of 31 feet is expected, 13 feet above flood stage.
President Bush declared a major disaster area in 22 Missouri counties. The declaration makes federal funding available to state and local governments and certain private nonprofit groups to help deal with weather and water damage.
U.S. Transportation Secretary Mary Peters promised $1 million in emergency relief for Iowa to help pay for urgent repairs to roads and bridges damaged by floods.
The state was drying out again after storms erupted Wednesday afternoon and continued into Thursday morning. In central Iowa, 5 inches of rain were reported in Polk City, near Des Moines, while Ottumwa in the southeast received over 2 inches of rain in about 30 minutes.
Meteorologist Marc Russell of the National Weather Service said rivers in southeast Iowa are still in the major flood category and any more rain, just an inch, could cause flash flooding. Russell says the same holds true for the Cedar Rapids and Iowa City areas, which suffered major flooding.
Iowa officials said water quality tests show that floodwaters heading down the Mississippi River contain large amounts of bacteria, sediment and fertilizers like phosphorus and nitrogen.
"When you have flooding of this scale, everything in the landscape is affected," said Iowa Department of Natural Resources spokesman Kevin Baskins.
So far, tests have not turned up large amounts of industrial chemicals or other volatile materials in the water, he said. Crews are locating and removing all the floating propane tanks or other floating canisters they can find.