DES MOINES, Iowa (AP) -- 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 Schomaker, 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.

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."

Later on Tuesday afternoon, city officials in Palo urged residents to evacuate and predicted flood levels as much as two feet higher than 1993 levels.

City officials said they would give 15 sandbags per house to residents until they run out, and said a relocation shelter had opened at Roosevelt Middle School in northwest Cedar Rapids.

Residents who leave town are being asked to call city hall to leave emergency contact information and place a white sheet or shirt on their door so officials know their house is empty.

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 water 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."