The breach of a main levee and subsequent failure of a temporary levee sent emergency crews scrambling Saturday as water from the Des Moines River rushed into a heavily populated residential area of Iowa's capital.

The Army Corps of Engineers and National Guard were ordering personnel to leave Des Moines just hours after a mandatory evacuation was issued for 270 homes. Officials warned of "significant property damage" of residential and commercial structures.

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Many Des Moines residents already had left after a voluntary evacuation request was issued Friday.

City crews and National Guard had been using dump trucks and front-end loaders to build a temporary berm in a bid to stop the water, but by midmorning they were ordered to abandon the effort as conditions worsened. The failure of the berm left hundreds of homes unprotected from flooding that had already surrounded the city's North High School.

"Things happened really fast," Toby Hunvemuller of the Corps of Engineers said. "We tried to figure out how high the level would go. Not enough time. We lost ground. We didn't want to risk life or harm anyone, and the decision was made to stop."

Since June 6, Iowa has gotten at least 8 inches of rain. That came after a wet spring that left the ground saturated. As of Friday, nine rivers were at or above historic flood levels. More thunderstorms are possible in the Cedar Rapids area over the weekend, but next week is expected to be sunny and dry.

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Bill Stowe, Des Moines' public works director, said he expected extensive damage to about 200 homes in the Birdland neighborhood. "There's not anything else we can do," Stowe said.

Water pouring through the 100-foot-wide gap in the main levee surrounded the city's North High School, and in places the water was up to 4 feet deep.

"We have a pretty strong gush of water coming through that area," said police Sgt. Vince Valdez, but despite the persistent flooding, the city was standing tough.

"We're still on the horse; we're doing good," he said. "We're going to enact any measure that we can to protect this city."

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In Cedar Rapids, at least 20,000 have been forced from their homes, and officials say it could be four days before the Cedar River drops enough for workers to even begin pumping out water.

The river crested Friday night at nearly 32 feet, 12 feet higher than the old record set in 1929.

Flood waters have swamped more than 400 blocks, threatened the city's drinking supply and forced the evacuation of a downtown hospital. Only one of the city's half-dozen wells was working, and it was protected by sandbags and pumps powered by generators.

"We're estimating at least a couple of weeks before the flood levels get down right around flood stage and below," said Dustin Hinrichs of the Linn County emergency operations center.

The damage in Cedar Rapids is extensive, with a preliminary estimate of $737 million.

"It's a bit overwhelming," said the city's mayor pro-tem, Brian Fagan. "This is an endurance competition. We have to be patient. We have to be cooperative."

Gov. Chet Culver has declared 83 of the state's 99 counties disaster areas, a designation that helps speed aid and opens the way for loans and grants.

Just south of Cedar Rapids, in Iowa City, Culver warned that more dramatic flooding could be on the way as the Iowa River rises.

"A real wave of water is on the way as we speak," he said.