Residents were ordered to evacuate low-lying sections of towns along the overflowing Cedar River on Wednesday, and communities along the Mississippi River were warned that new rainfall would boost their expected flood crests.
Officials in Wisconsin, where this month's rainfall is approaching a record, planned to drain water from one reservoir to ease pressure on a dam, and were monitoring dams elsewhere in the state. High water in Indiana burst a levee Wednesday and flooded a vast stretch of farmland.
A new wave of rain showers spread across parts of Iowa on Wednesday, including some flood-threatened areas. The rain came as a band of storms rippled across the northern Plains.
As the rain fell and the rivers rose, vast stretches of Iowa simply went under, reports CBS News Correspondent Dean Reynolds from Cedar Rapids.
"It's unbelievable to say the least," said one man in Waterloo, Iowa. "It is shocking."
"I always wanted to live on the ocean," one Waterloo woman joked. "But not like this."
Across Iowa, entire blocks of residential streets disappeared. Highways were closed for good reason, bridges were overtopped, and all you could see of some communities were the tips of their houses. Iowa's neighbors were hardly better off.
In Indiana, 44 counties are now sodden disaster areas. And along the swollen Wabash River more than a thousand national guardsmen have been called out to help.
In Illinois, broken levees plunged thousands of acres of farmland under water and homes went right along with them.
In Wisconsin, half the state is flooded. Officials are monitoring dams, and an emergency has been declared in 30 counties.
Torrential rains like those today over the entire Midwest have brought up memories of 1993, another record setting rainfall season. The difference now is that people have learned how to manage it - at least to some extent.
Downtown Des Moines was under water Wednesday - but this time it was on purpose - and on schedule, reports CBS News weatherman Dave Price from Des Moines.
Overnight the Army Corps of Engineers slowly lowered six-foot emergency flood gates at Saylorville Lake, a man-made reservoir on The Des Moines River. This allowed a controlled flood, rather than a sudden rush of water, Price reported.
Even so, water was pouring out at as quickly as 314,000 gallons a second.
Manhole covers erupted as water rushed up into city streets from the sewer systems below.
Water rushed in from the sides too, as river surged into the city.
It was all too reminiscent of the '93 floods - when six states were declared disaster areas and 70,000 people were left homeless.
Since then, this inflatable gate system at Saylorville Lake was built. It is now being used for the first time.
Meanwhile, in Cedar Falls, which is to the northeast of Des Moines, a sandbagged levee prevented the Cedar River from flooding the city on Wednesday, but officials asked for extra volunteers to help shore it up. Just downstream along the Cedar River, the neighboring city of Waterloo ordered a mandatory evacuation of some neighborhoods, not because of the river but because the ground was saturated and pumping stations couldn't keep up, officials said.
To the southeast in Cedar Rapids, more than 200 residents of a neighborhood near the river were told to seek higher ground.
In Vinton, electricity was cut Wednesday morning when rising water affected the city's municipal power plant, said Steve Meyer, the assistant emergency operations center manager. He said a 15-block area near the river had already been evacuated.
"The water is at least 3 feet deep. It's still coming up," he said of the town, home of about 5,000 people between Cedar Falls and Cedar Rapids.
The Cedar River had been expected to top the Cedar Falls levee during the night and deluge the downtown area of the city of 35,000 people some 88 miles northeast of Des Moines. But city spokeswoman Susan Staudt said early Wednesday that the sandbags appeared to be holding.
Flood stage at Cedar Falls is 88 feet, and by about 5 a.m. the river stood at 101.8 feet, down slightly from earlier in the night. The previous record was 99.2 feet in 1999.
Thousands of volunteers who showed up Tuesday to help with the sandbagging effort "saved this city, but we are still at a critical point," Staudt said.
"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."
Greg Starbeck said Wednesday that he and his three daughters helped fill and stack sandbags until late Tuesday.
"Everybody is doing something, whether it's tying bags or getting water for other people," Starbeck said. His house was safe from flooding but water was 5 feet deep in a rental house he owns.
Rising waters also threatened Palo to the south. City officials there urged residents to evacuate, predicting flood levels as much as 2 feet higher than 1993 levels, which left much of the state under water.
City officials said they would give residents 15 sandbags per house until they run out. They opened a shelter in nearby Cedar Rapids and asked residents who leave town to call City Hall to leave emergency contact information and to place a white sheet on their door so officials would know their house was empty.
Floodwaters were threatening water treatment plants in several towns, Lt. Gov. Patty Judge said. Mason City's plant was knocked out of service Sunday after the Winnebago River broke through a levee, while officials in Des Moines hoped that releasing water from the Saylorville Reservoir would protect the capital city and its water treatment plant from flooding.
Along the Mississippi River, the National Weather Service has predicted crests of 10 feet above flood stage and higher over the next two weeks in the Missouri and Illinois stretch of the river. Most of the towns are protected by levees, but outlying areas could be flooded.
The river was 1.5 feet above flood stage Wednesday at St. Louis, on its way to 5.6 feet above, and the floating President Casino closed for the second time this year because of flooding on its riverfront access road.
With more heavy rain forecast upriver in Iowa, crests at Missouri's Mississippi River towns could be higher than currently forecast, said weather service hydrologist Jim Cramper.
"It's when the rain falls upstream that it's a bigger impact on you," Cramper said. "As this new batch of water comes down, the river could start creeping up again."
That reflects conditions that have existed all spring, said Susie Stoner, spokesman for Missouri's State Emergency Management Agency .
"We have rivers that have been at flood stage since March," Stoner said.
In Wisconsin, state and local officials were monitoring dams threatened by the high water from days of storms.
Wisconsin officials planned to let water out of the reservoir behind Primmer Dam, in the southwestern part of the state, to ease pressure on the weakening structure, said Russell Rasmussen of the Department of Natural Resources. He said a number of homes and a highway are located downstream from the dam.
"There's some seepage coming out," Rasmussen said. "We're concerned the dam is weakened to the point of becoming a problem."
As of Wednesday morning, Milwaukee had measured 9.21 inches of rain already this month - including a record 7.18 inches in a 48-hour period - and the June record is only 10.03 inches, set in 1917.
A levee failed early Wednesday in southwest Indiana near the town of Capehart, flooding several square miles of farm land near the White River, and Daviess County authorities urged residents to evacuate.
"We've got about a 40-yard swath of levee that's gone," said Indiana state Rep. Dave Crooks, speaking for the county's Emergency Management Agency. "We've got rapidly rising water in that whole bottom area."
Authorities also ordered as many as 300 residents north of nearby Maysville to evacuate late Tuesday after water topped a levee.
Elsewhere, thunderstorms brought relief to parts of the East Coast that have been baking in a heat wave for four days. Temperatures in the upper 90s on Tuesday stretched from Georgia all the way to northern New England, where the weather service reported an afternoon high of 99 at Portsmouth, N.H.
The storms also knocked out power to hundreds of thousands of homes and businesses. About 103,000 customers in New Jersey, 110,000 in southeastern Pennsylvania, 50,000 in upstate New York and 20,000 in Connecticut remained without electricity early Wednesday.
Philadelphia officials blamed the deaths of two women on the heat wave.