Floodwater from the Bourbeuse River covers a neighborhood Tuesday in Union, Mo. In addition to threatening homes, the floods have closed miles of roads.

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.

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, Mo., about 20 miles north of St. Louis. Mayor William Richter ordered any of the about 520 residents who had not already evacuated to get out town.

Across the river, in Alton, Ill., dozens of volunteers helped place sandbags. Mayor Brant Walker said 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 several businesses.