(AP Photo by Christopher Gannon, Des Moines Register) Cruel irony came in the form of a street sign as Tony Malone pushes a catamaran to help neighbors salvage items from their flooded home on Water Street Friday morning, June 6, 2008, in Cambridge, Iowa. Water from the swollen South Skunk River began rising into homes. Runoff from heavy rains overnight swamped the low-lying area in the central Iowa town, forcing the evacuation of about 12 homes.

DES MOINES -- With rivers continuing to rise and more heavy rain on the way, state officials said Tuesday they are trying to help towns already battered by floodwaters while working to protect others.

The rising Cedar River was causing the most concern in Cedar Falls, where officials were preparing residents and downtown business owners to evacuate as the river threatened to spill over a levee that protects the area.

Mayor Jon Crews said the river is expected to top the levee early Wednesday, prompting a mandatory evacuation of the downtown. The river was expected to crest at a record 15 feet above flood stage at 1 a.m. Wednesday.

"I've been downtown for 37 years and I have never seen anything like this," said Steve Shoemaker, a partner in a local insurance company. "This is incredible."

Donita Krueger was among those helping fill sandbags in hopes of holding the water back.

"If this breaks, the whole downtown will be flooded," she said. "Everything goes on down here. It would be a big hit to the community."

White, yellow and orange sandbags lined the downtown. Tarps and plastic were taped up to windows and doors.

AP Photo by Matthew Holst, Iowa City Press-Citizen Ryan Strang works to strengthen a sandbag wall in front of The Cliff Apartments on Monday, June 9, 2008, in Iowa City, Iowa. Iowa Gov. Chet Culver said nearly a third of his state's 99 counties need federal help for flooded areas.

As Cedar Falls waged its fight against the river, the high water also was threatening Palo to the south, said Lt. Gov. Patty Judge during a news conference updating the status of flooding across the state.

"Everyone is working very, very hard and I want to be certain that the people of Iowa understand this is a serious situation and they should be prepared," Judge said.

She said people shouldn't be alarmed, "but I do believe it may be necessary to do evacuations of a large number of people."

In Waterloo, the fast-moving water swept away a railroad bridge used to transport tractors from the John Deere Tractor Works to Cedar Rapids. It also prompted the city to shut its downtown and close five bridges into the district.

The Des Moines and Raccoon rivers in central Iowa were being watched closely as officials worked to protect downtown Des Moines and the city's water treatment plant. Officials planned to close most of the downtown river bridges at 6 p.m. Tuesday because of flooding.

Officials in Des Moines believe the city's levees can contain the rapid release of water from the Saylorville Reservoir, scheduled to begin Tuesday night, said Bill Stowe, the city's public works director.

The Saylorville reservoir, which has been filled by heavy rainfalls to the north, will double its outflow to 42,000 cubic feet per second Tuesday night.

The Des Moines River in downtown Des Moines is expected to crest at as much as 8 feet above flood level Wednesday evening.

More rain is in the forecast for Wednesday and Thursday and the impact won't be known until after the National Weather Service runs flood projection models, said Roger Less, of the U.S. Army Corps of Engineers.

Stowe said city officials will closely watch those projections.

"As we look at higher precipitation ... obviously we become much more concerned," he said.

Dave Miller, administrator of Iowa Homeland Security and Emergency Management, said changes in river forecasts have left officials expecting worse flooding.

"Things are moving a little quicker than we anticipated," he said. "What that means is higher than predicted flows and coming a little quicker downriver."

He compared the current situation to record flooding in 1993, which left much of the state under water.

"The impacts may not be the same as '93, but the severity of the waters, the record or near record crests that are going to happen throughout the state means we are in severe flooding in a number of areas and we will continue to be throughout this week and perhaps into next week," Miller said.

Miller and Judge said the Iowa National Guard and the Iowa State Patrol are prepared to offer help anywhere they're needed for flood control, security and rescue.

"This is an extremely serious and challenging situation, but one that we are working to try to stay on top of and I think we're doing that -- I think we're ahead of it," Judge said.

Miller said about 530 people have been evacuated from eight communities, including New Hartford, Nashua and Decorah. About 30 people are in six shelters that have been set up.

The rising waters was threatening water treatment plants in several towns, Judge said. Mason City's plant was knocked out of service on Sunday after the Winnebago River broke through a levee there.

Judge said the Duane Arnold nuclear power plant in Palo shouldn't be affected by the flooding.

"We believe ... that it will be in good shape even if there is severe flooding -- the plant will be able to sustain its operation."

* Associated Press writers Amy Lorentzen in Cedar Falls and Luke Meredith in Des Moines contributed to this report.

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Waterloo railroad bridge swept away by flood waters

WATERLOO (AP) - A portion of a railroad bridge that crosses the Cedar River in Waterloo has been swept away by raging flood waters.

About one-third of the bridge washed away around 2:45 p.m. on Tuesday. A portion of the bridge struck a street bridge and another piece washed downriver.

The bridge is used by the Iowa Northern Railroad to transport tractors from the John Deere Tractor Works to Cedar Rapids.

A railroad spokesman says the railroad is assessing the affects of the bridge washout.

New Hartford residents unsure about future

NEW HARTFORD (AP) -- Only two weeks ago, it was a deadly tornado.

Earlier this week, it was flooding.

The twin disasters have left the 650 residents of New Hartford unsure what will become of their northeast Iowa community.

"Flooding is nothing new to New Hartford ... but just this time it was too much," said Corey Woods, a metal spinner and security guard. "It was relentless, no holds barred I guess for the weather."

Flooding poured over a dike and swamped much of the south side of town late Sunday, inundating residents who were just starting to recover from a May 25 tornado that killed two people. More than 150 people had to be rescued from their homes.

"We're just discouraged, and a little angry," said Woods, taking a break from cleaning his parents' water-logged home.

Residents said they hoped to rebuild but were unsure of the town's future after two hard hits.