Hundreds of Iowans fled from their homes and vacation cabins after two days of heavy rains caused the Lake Delhi dam to rupture on Saturday.
Authorities said the torrential rains - 15 inches in 48 hours - washed out a 125-foot wide, 40-foot deep section of earth.
"It just peeled off eight-foot sections and dumped them," Delaware County supervisor Shirley Helmrichs said the light polls fell like "match sticks."
"They just started snapping over. You could hear this crunching, this rumbling. It was like the dam was just growling," she said.
Helmrichs said boats, docks and trees shredded. "It took just seconds to shuck them through from perfect to tiny crumbs...it just tumbled down, slow motion, into the river. It was just so eerie," she said.
Delhi Fire Department firefighter Jeremy Sands said the 83-year-old dam was not unsafe. "It's just one of those acts of God," Sands said, according to The New York Times.
Delaware County emergency manager Mike Ryan called the breach "the worst damage" he had ever seen.
North of the dam in Manchester, the flood crested at a record high of 24.5 feel, beating the 2004 record of 21.66 feet, according to Ryan, who predicted that disaster assessment "is gong to be a nightmare."
Meanwhile in Chicago, expressway traffic plowed through standing water after more than 7 inches of rain fell early Saturday, flooding sewer systems and waterways.
Water covered portions of Interstate 290, west of downtown Chicago, and commuter train tracks. City crews diverted traffic and called in busses to shuttle some passengers to their destinations.
In suburban Westchester, rescue workers searched for people stranded in flood homes or trapped in their cars under viaducts.
Chicago Mayor Richard Daley urged residents to call for help.
"Our goal is to get the city back to normal as quickly as possible," the mayor said a press conference on Saturday.
The New York Times and AP contributed to this report.