(Updates with tornado hitting children's camp)

By Kay Henderson

DES MOINES, Iowa, June 11 (Reuters) - A tornado ripped through a boy scout ranch in Iowa on Wednesday injuring at least 20 people, local officials said -- one of 28 reported in the U.S. Midwest.

Several sources, including a local weather office and a state official, reported four killed at the Little Sioux Boy Scout Ranch near Sioux City in western Iowa, but the Monona County Sheriff's Office would not confirm any fatalities.

Other twisters struck eastern Kansas, Nebraska, Iowa and Minnesota, according to Storm Prediction Center meteorologist David Imy.

The tornadoes were accompanied by baseball-sized hail and vicious winds, and came on top of rampant flooding that has forced hundreds from their homes in Iowa alone.

Iowa Gov. Chet Culver on Wednesday declared 54 of Iowa's 99 counties disaster areas due to damage from the flooding and tornadoes. He said this year's spring deluge is comparable to the disaster of 1993 when the Mississippi River and its tributaries turned parts of the nation's midsection into a gigantic lake.

Culver predicted the damage will run in the "hundreds of millions of dollars."

"We have nine rivers right now across this state, not including the Missouri and the Mississippi, that are above record level or very close to record level," Culver said. "That's all time records."

Heavy rains in Des Moines flooded restaurants and a government building in the business district and water released from a reservoir was expected to swell the Des Moines River that bisects the city.

In Cedar Rapids, some 800 residents were evacuated from their homes on Wednesday.

Donita Dettmer's hilltop home on the edge of Waverly remained dry, though she said she had never before seen the river submerge the adjacent highway.

"We're fairly lucky," Dettmer said, in contrast to scores of neighbors displaced by the flooding.

CROP PRICES SOAR

Corn prices hit a record high as meteorologists predicted another batch of thunderstorms would douse sodden farmland later this week.

"Even if it is drier next week it won't matter now. It's too late to plant corn," said Vic Lespinasse of grainanalyst.com.

The U.S. Army Corps of Engineers announced it was closing locks and dams on 200 miles (323 km) of the upper portion of the Mississippi River, a vital commercial waterway, possibly through early July.

Tyson Foods Inc shut down its pork slaughtering plant at Columbus Junction, Iowa, because flood waters prevented workers from reaching the facility.

Dams across the Midwest were being monitored for signs of weakening and spillways on many reservoirs were overtopped.

Tens of thousands of people were without power in Michigan, Wisconsin, Indiana and Illinois since violent weekend storms.

The White River in central Indiana breached a levee guarding the town of Capeheart, forcing out residents.

The 9,000 people in the village of Pewaukee, Wisconsin, were staying put for now, though the rising waters of Lake Pewaukee had swallowed up the town's beach and was lapping up against lakeside restaurants.

There was a stench from dead fish that swam onto flooded village streets from the expanding lake.

"We've been out there with pitchforks, shovels, even our hands, catching hundreds of them," village clerk Susan Atherton said by telephone. (Additional reporting by Carey Gillam, Editing by Alan Elsner)

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