ST. LOUIS — Federal officials were monitoring 19 vulnerable levees on the rising Mississippi River and its tributaries, warning that hundreds of homes in Illinois and Missouri 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, Illinois’ governor declared disasters in seven counties and 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.

The Meramec River near St. Louis was expected to get to more than 3 feet above the previous record by late this week.

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

The river on Tuesday spilled over the top of the levee at West Alton, Missouri, about 20 miles north of St. Louis. 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.

Mayor Brant Walker said in a statement that “even with our best efforts of sandbagging and pumping,” flooding was expected at least in the basements of the downtown business district.

In another eastern Missouri town, Union, water from the normally docile Bourbeuse River reached the roofs of a McDonald’s, QuikTrip and several other businesses. The river reached an all-time high Tuesday, nearly 20 feet above flood stage.

Interstate 44 was closed near the central Missouri town of Rolla, and a 10-mile section of Interstate 70 was shut down in southern Illinois before it was reopened late Tuesday afternoon. Hundreds of smaller roads and highways were also closed across the two states, and flood warnings were in effect.

Missouri Gov. Jay Nixon activated the National Guard to assist with security in evacuated areas and to help keep road closure sites clear.

In southern Illinois, the Department of Corrections transferred an unspecified number of inmates from a state prison to other locations because of flooding risks. The facility houses nearly 3,700 inmates.