ST. LOUIS: Federal officials were monitoring 19 vulnerable levees on the rising Mississippi River and its tributaries, warning that hundreds of homes in the Midwest could be threatened by a rare winter flood that already forced the partial closure of interstate highways and widespread evacuations.

As the swollen rivers and streams pushed to virtually unheard-of heights Tuesday, an unknown number of inmates were transferred out of an Illinois state prison threatened by flooding and the state's governor declared disasters in seven counties. Missouri's governor activated the National Guard to help divert traffic from submerged roads.

Record flooding was projected in some Mississippi River towns after several days of torrential rain that also caused sewage to flow unfiltered into waterways.

At least 20 deaths over several days in Missouri and Illinois were blamed on flooding, mostly involving vehicles that drove onto swamped roadways.

The Mississippi River is expected to reach nearly 15 feet (4.5 meters) above flood stage on Thursday at St. Louis, which would be the second-worst flood on record, behind only the devastating 1993 flood.

The river on Tuesday spilled over the top of the levee at West Alton, Missouri. Mayor William Richter ordered any of the town's approximate 520 residents who had not already evacuated to get out of harm's way.

Across the river, in Alton, Illinois, dozens of volunteers helped place sandbags ahead of where water is expected to rise.

In another eastern Missouri town, Union, water from the normally docile Bourbeuse River reached the roofs of several businesses.

The high water was blamed on the shutdown of a wastewater treatment plant on Monday just south of St. Louis, causing sewage to go directly into nearby rivers and streams.

One of the two wastewater plants in Springfield, Missouri, also failed, allowing partially treated sewage to flow into a river.

The US Coast Guard closed a 5-mile (8-kilometer) portion of the Mississippi River near St. Louis due to flooding. Capt. Martin Malloy cited high water levels and fast currents in the river, which is a vital transportation hub for barges that carry agricultural products and other goods.

In Granite City, Illinois, about 30 residents of a flooded trailer park idled in a Red Cross emergency shelter in a church basement. The park's property manager told Shirley Clark (56) and other displaced residents that it could be another 10 to 12 days before they're able to return to their homes.

"We need help over here," said Clark, a diabetic who said she left behind her insulin supply. "We're just holding on."