CEDAR RAPIDS, Iowa — Displaced residents trickled back into the hardest-hit areas of Cedar Rapids today for their first up-close look at flood devastation, while a forecast of an earlier and lower flood crest at the Iowa River sparked hope that a university city would escape a similar fate.

The National Weather Service had predicted a 33-foot crest early Tuesday atIowa City, home of the University of Iowa, but the latest projection today showed the Iowa River is expected to top at about 31.5 feet and hold there before starting to fall tomorrow evening.

At a Des Moines press conference, Governor Culver called it “a little bit of good news” but said the situation was still precarious.

“Just because a river crests does not mean it’s not a serious situation,” he said. “You’re still talking about a very, very dangerous public safety threat.”

A weather service meteorologist, Donna Dubberke, said levee breaks downstream on the Iowa River might explain the lower crest.

“We believe that some of that water is able to go off in those areas and that’s just provided some extra storage,” she said.

More than 20 buildings at the University of Iowa had already taken on water, but the lower crest probably would spare dozens of others that had been in jeopardy, a Johnson County Emergency Management spokesman. Mike Sullivan. said.

In Cedar Rapids, the Cedar River had gone down 5 feet from its record crest at 31.1 feet.

“As the river recedes we’re beginning to see the incredible destruction that is left behind,” a spokesman for the city of Cedar Rapids, where the worst flooding has taken place, Dave Koch, said.

Houseboats torn from their moorings by the current were smashed against a partially collapsed railroad bridge and fish were occasionally flopping on city streets. High water marks on buildings were visible 5 feet above the ground or higher, and formerly submerged cars were beginning to emerge from the water.

Warnings about the dangers of walking in the polluted, debris-strewn water prompted hundreds of people to line up at a downtown clinic this morning for free tetanus shots.

The city planned to set up 10 checkpoints where residents will have to show ID and sign in before being allowed to return home to begin removing belongings. Houses where damage is severe will remain off limits, Mr. Koch said.

Mr. Koch said additional National Guard troops were being deployed to the city to help secure the perimeter around the flood area.

He said it would be two to three more days before the river drops enough so crews can begin pumping water back over the levees, and another week before the river falls below flood stage.

The flooding at Cedar Rapids swamped 1,300 city blocks and forced 24,000 people from their homes.

At least three deaths in Iowa have been attributed to the flooding and storms that caused it, and 12 others have died in two recent tornadoes.

Elsewhere in the Midwest, hundreds of members of the Illinois National Guard headed to communities along the swollen Mississippi River today for sandbagging duty while emergency management officials eyed rain-swollen rivers across the state.

Two levees broke yesterday near the Mississippi River town of Keithsburg, Ill., flooding the town of 700 residents about 35 miles southwest of Moline. The National Weather Service said the Mississippi would crest Tuesday morning near Keithsburg at 25.1 feet. Flood stage in the area is 14 feet. Rising water threatening approaches also prompted Illinois officials to close a Mississippi River bridge at Quincy.

A levee break along the Iowa River in the southeastern corner of the state swamped tiny Oakville, population 439. Also in southeast Iowa, authorities told all the roughly 250 people at Fredonia to leave their homes and ordered more evacuations in two other small towns, Columbus Junction and Columbus City — all clustered near the junction of the Iowa and Cedar rivers.

The municipal water system at Cedar Rapids was back to 50% of capacity today, a big victory after three of the city’s four drinking water collection wells were contaminated by murky, petroleum-laden floodwater. That contamination had left only about 15 million gallons a day for the city of more than 120,000 and the suburbs that depend on its water system.

The city utility director, Pat Ball, said today that residents can shower and use toilets but they’re still being asked to be very careful about how much water they use.

About 5,000 residents already have been displaced in the Iowa City area.

One of the biggest concerns was the University of Iowa. The Iowa River splits the campus and damage has already been enormous. More than 20 buildings had taken on water, including the art museum, a recital hall, and other buildings on what’s known as the Arts Campus.

Several hundred people turned out to help campus sandbagging efforts.

“The volunteers have just been incredible,” Mr. Sullivan said. “Some of these people, they’ve lost their homes, they’ve lost their businesses, but they’re still down there helping.”