DES MOINES, Iowa — Rising rivers wiped out an Iowa railroad bridge yesterday, flooded Illinois farmland, and forced residents along the Mississippi River to prepare for what could be the worst flooding in 15 years.

In Cedar Falls, Iowa, officials were readying residents and downtown business owners to evacuate as the Cedar River threatened to spill over a levee. The river was expected to top the levee early toady, prompting a mandatory evacuation of downtown, Mayor Jon Crews said.

“I’ve been downtown for 37 years and I have never seen anything like this,” a partner in a local insurance company, Steve Schomaker, said.

In nearby Waterloo, fast-moving water swept away a railroad bridge used to transport tractors from a John Deere factory to Cedar Rapids. It also prompted the city to shut its downtown and close five bridges.

Levee breaks yesterday in southeastern Illinois flooded between 50 and 75 square miles of farmland along the Embarras River, forcing the evacuations homes northeast of Lawrenceville, the Lawrence County sheriff, Russell Adams, said. He said water was up to the roofs of some rural homes.

In Elnora, Ind., about 100 miles southwest of Indianapolis, berms of white sandbags and concrete barriers held back the White River, leaving residents little choice but to wait and watch. Most residents left after voluntary evacuation orders came late Monday, two days after the area got up to 10 inches of rain.

“We have a very touch-and-go situation there, but everything that can be done has been done,” Indiana’s governor, Mitch Daniels, said.

Along the Mississippi River, the National Weather Service yesterday predicted crests of 10 feet above flood stage and higher over the next two weeks. Most of the towns are protected by levees, but outlying areas could be flooded.