CEDAR RAPIDS, Iowa (CNN) -- Floodwaters inundated Iowa City and the University of Iowa arts campus on Sunday despite what one official called a "Herculean effort" to hold back the water with sandbags.
"We've had the [National Guard] working next to prisoner inmates, sandbagging," said David Jackson, the university's facilities manager. "Students, faculty and staff, leaders of the university, the president of the university -- out sandbagging."
Some 500 to 600 homes were ordered to evacuate and others faced a voluntary evacuation order through the morning, said Iowa City Mayor Regenia Bailey.
The Iowa River in Iowa City crested at 31.5 feet and was expected to remain at that level until Monday, city and state officials said Sunday.
Classes at the university have been suspended until next Sunday, according to its Web site.
"All of our theaters, our music building, Clapp Recital Hall, our fine arts building [the] new Art Building West designed by Stephen Holl, has taken on significant water as well," said Sally Mason, president of the university. "Fortunately we were able to save all the art," she said.
The art was placed in crates shipped out of state last week.
"We anticipated the worst a week ago." At least 8 feet of water rushed through the campus, officials said. Among the school's 30,000 students, Ann Barber told CNN she has been sandbagging for nearly seven days.
"It's very hard to watch the devastation of our university," she said.
This month's severe weather has trampled towns from North Dakota to Indiana. The Federal Emergency Management Agency says more than 11 million Midwesterners will be affected by flooding and tornadoes.
Meanwhile, in Cedar Rapids, Iowa, some of nearly 20,000 displaced residents began to return home Sunday as water there receded.
People lined up for about a block in one part of the city waiting for a special wristband to allow them access to their homes. The flooding there caught many people by surprise.
"We didn't think it would get this high," said Tina Fleischacker, whose Cedar Rapids home was soaked. "We moved everything upstairs and it's gone. It's gone. We left with the clothes on our backs."
About 36,000 Iowans, most in Cedar Rapids, evacuated their homes due to the state-wide flooding. At least 472 people spent Saturday night in 18 shelters set up across the state, according to Dave Miller, the administrator of Iowa Homeland Security and Emergency Management.
In Iowa City, the water is expected to drop no more than 3 feet by Saturday, said John Benson, spokesman for Iowa Homeland Security and Emergency Management.
"There's that moment of 'phew,' but then there's that realization that the water will be going down very slowly," Bailey told reporters.
She urged residents to be careful when returning to their homes and businesses, and asked them to abide by a 8 p.m. to 6 a.m. curfew.
"Water flows are still dangerous," she said. "We need people to be patient. We will get them into those homes and businesses as soon as possible."
Iowa has been inundated with heavy rains in recent weeks that have caused several major rivers that feed into the Mississippi -- including the Cedar, Des Moines and Iowa Rivers -- to flood their banks.
The flooding in the Midwest is "some of the worst" to hit the United States since Hurricane Katrina inundated the Gulf Coast nearly three years ago, FEMA administrator David Paulison said Sunday on CNN.
The scenarios are much different, but "the aftermath is similar," he said. "The fact [is] that we have a lot of people whose homes have been destroyed."
The agency has received more than 12,000 disaster assistance applications from the hardest-hit states -- Iowa, Indiana and Wisconsin.
Starting Tuesday, the American Red Cross will set up kitchens in Iowa to serve up about 100,000 meals to residents each day. The agency, which is housing 720 flood victims in 30 shelters, plans to spend about $15 million on Midwest relief efforts.
Iowans are very concerned about how they will afford to rebuild.
"Most of the people here ... do not have flood insurance," said Steve Doser, director of a shelter in Cedar Rapids.
"A couple people told us ... that they were told they didn't need flood insurance, 'Don't worry about it, you're in a 500-year [plain],' " he said. "Now they don't have anything."
Iowa Gov. Chet Culver estimates agricultural damage could reach $1 billion, exceeding the costs of the big flood in 1993. He praised the strength and resilience of the people of Iowa and vowed to rebuild the state, noting that "will take a long time."
There have been 16 storm-related deaths since May 25 in Iowa, 12 of them from recent tornadoes, Culver said Sunday. Four Boy Scouts were killed last week when a twister touched down at a camp in Iowa.
Culver has declared 83 of the state's 99 counties disaster areas. More than 3,300 Iowa National Guard troops have been deployed to help primarily with sandbagging and staging resources, Maj. Gen. Ron Dardis of the Iowa National Guard said Sunday.
That number is expected to rise to 4,000 by Monday, he said. Of those troops, 750 are stationed in Des Moines helping to shore up levees with sandbags along the Des Moines River amid fears that the historic flooding that has hit other parts of the state could soon take its toll on the Iowa capital.
Early Saturday, rising waters breached a levee on the Des Moines River, prompting emergency officials to evacuate 270 homes in Des Moines' Birdland Park neighborhood, a state emergency official said. A high school in the neighborhood was also flooded.
CNN's Jim Acosta and Julian Cummings contributed to this report.