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ST. LOUIS • Water rushing down the Missouri River from swollen reservoirs in the Great Plains won't break area levees, but the added power of widespread heavy rain probably would, federal floodwatchers warned Thursday.

Any such breaks would be of agricultural levees, swamping bottomland farms. The big levees protecting Chesterfield, Maryland Heights, West Alton and other developed areas would hold.

That picture was offered by the National Weather Service and the Army Corps of Engineers in a news conference called partly to calm doomsday fears inspired by images of levee breaks in northwestern Missouri. Normal rainfall spread evenly won't be serious trouble, but officials warned of major flooding if heavy storms pound the river's wide basin.

On Tuesday, the corps opened the Gavins Point Dam, near Yankton, S.D., to five times normal flow because the massive flood-control reservoirs on the upper Missouri are at brimful, compliments of heavy snowmelt and record rain in Montana and the Dakotas. Gavins Point is the last of five dams on the river.

That water is expected to reach St. Charles late next week and be enough to keep the river about 3 feet over flood stage through August. Normally, it takes 10 days for water at Gavins Point to reach St. Louis.

"Because of that flow coming down the river, we will be close to flood stage all summer. That's the new normal," said Wes Browning, chief of the weather service office in Weldon Spring. "But if we get much above normal rainfall, or big bursts of rain, there's likely to be trouble."

Browning's "new normal" at St. Charles is 9 to 14 feet above the normal summertime levels on the lower Missouri.

Browning and Col. Tom O'Hara, commander of the corps St. Louis district, showed maps of potential flooding on the lower Missouri and Mississippi. They show that widespread heavy rain would boost the Missouri to 12 feet over flood at St. Charles.

That would be only three feet shy of the crest during the Great Flood of 1993, enough to break most of the agricultural levees on the lower Missouri.

"The water from the reservoirs has not produced, and will not produce, overtoppings in the St. Louis area," O'Hara said. "If we get (heavy) rainfall, we could have issues with some of the levees."

He said the high water that has broken some levees in northwestern Missouri won't have the same effect here because the lower Missouri "has more capacity to absorb that flow."

Browning said the five-day forecast calls for heavy rain in the upper Missouri and upper Mississippi river basins. The long-range outlook, issued Thursday, predicts above-normal chance for rain across the upper reaches of the Missouri's watershed.

"That could be serious," Browning said.

Browning said normal rainfall amounts, if dumped in concentrated bursts, can create major floods. He compared it with giving a lawn 1 inch of water by sprinkler overnight or by fire hose in five minutes. "You get very different outcomes," he said.

Dave Garrison, of the St. Charles County Emergency Management Agency, said a major flood "would be devastating to farmers, with thousands of acres underwater."

Recent heavy rain to the north is also boosting the Mississippi River, which has been high for weeks. In Hamburg, Ill., population 128, across from Elsberry, volunteers ringed houses with sandbags Thursday, said village clerk Koni Proctor.

Upriver at Clarksville, Mo., where crews built a temporary floodwall in April, the city has 100,000 sandbags "ready if we need them," said Mayor Jo Anne Smiley.

If the current crest forecast holds true, she said, the bags will remain on standby.

At St. Louis, the river is expected to crest 3 feet over flood stage Sunday and begin falling.