By Ryan Schlader

CEDAR RAPIDS, Iowa, June 14 (Reuters) - A levee holding back rising floodwaters broke and swamped Iowa's capital on Saturday as officials across the U.S. Midwest reinforced levees, helped displaced residents and cleaned up the mess left by the region's worst flooding in 15 years.

The region got a break from the torrential rains and high winds, but some states braced for more rain, which was forecast for Saturday night.

"Thankfully we're having some dry weather right now," said Alan Foster, a spokesman for Iowa's state emergency management office. "But with everything that's flowing ... we're still just hanging on."

Iowa was the epicenter of the flooding that swamped Kansas, Missouri, Wisconsin, Minnesota, Michigan and Indiana.

In Des Moines, Iowa's capital, a levee holding back the Des Moines River broke overnight, sending water rushing into a neighborhood near downtown.

In Cedar Rapids, the state's second-largest city, the waters of the swollen Cedar River crested overnight. But more than 400 city blocks remain waterlogged and 24,000 people have been forced from their homes.

Mayor Brian Fagen toured the downtown streets by motorboat. He said the city's main fire station was submerged and he saw "block upon block" of devastation.

"Nobody expected this and planned for this," resident Phyllis Hikey said. "I thought we were pretty safe, but no, we weren't."

Downstream in Iowa City, rising waters threatened the University of Iowa, the city's largest employer. A U.S. Coast Guard team was evacuating residents.

MORE RAIN PREDICTED FOR CENTRAL IOWA

Iowa Gov. Chet Culver has declared 83 of the state's 99 counties disaster areas and dispatched 2,500 National Guard troops to flooded communities.

The National Weather Service was predicting another quarter- to half-inch of rain for central Iowa, putting renewed pressure on the region's stressed dams, levees and bridges.

In Illinois, Gov. Rod Blagojevich activated 200 National Guard troops and other state aid workers to help evacuate residents of Carman and Keithsburg, two towns on the upper Mississippi River where levees broke on Saturday morning.

Blagojevich declared seven counties along the Mississippi River threatened by the rising waters disaster areas.

The White House issued a statement on Saturday promising to provide victims with shelter and low-interest loans.

In Quincy, Illinois, Democratic presidential candidate Barack Obama joined volunteers and filled sandbags to hold back the Mississippi River. Republican presidential candidate John McCain issued a statement expressing sympathy for the flood victims.

Culver said the damage to his state could total billions of dollars. Scores of bridges spanning nine overflowing rivers have been swept away or weakened. Many highways remain closed, disrupting interstate truck transportation and tourism.

The flooding led authorities to close the upper Mississippi River to barge traffic. Commerce on a 300-mile (480-km) stretch of the most important U.S. waterway may be halted for weeks.

Iowa is usually the top U.S. corn- and soybean-growing state and a major producer of hogs and cattle. The washed out fields and reduced harvest outlook has sent crop prices soaring, with corn prices at the Chicago Board of Trade setting a record for the seventh straight day in Friday.

Crop losses could increase prices for food and fuels, like corn-based ethanol, and feed growing fears of inflation.

(Additional reporting by Kay Henderson in Des Moines)

(Writing by James B. Kelleher)

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