They're calling it the 500 year flood. As the rains fell all over the midwest, the ground already saturated from record snowfalls couldn't absorb any more water. Rivers ands creeks rose to levels never seen before.
In the end 25,000 people were without homes, 1300 city blocks were under water. It's the worst flood in Iowa history.

Here's some footage from when the water was about to crest.