FENTON, Mo. — Amid the worst flood this town has ever seen, Tammy Morgan took a break Wednesday from shoveling sand into the yellow bags she hoped would save her business and pointed to a line of treetops rising from the water. There, she said, a few hundred feet away, past streets and clapboard houses now submerged, is where the Meramec River should be — not lapping at the grass a few feet away.

“It’s pretty far away,” she said. “It’s a lot to get up here, you know?”

Some of the highest flood stages ever recorded hit the Mississippi River basin, part of a band of severe flooding stretching from northern Texas to the Ohio River Valley, affecting millions of people. Neighborhoods have been evacuated, towns inundated, roads and water treatment plants closed.

In Illinois and Missouri, where the governors declared states of emergency across wide swaths of the states, officials had blamed 20 deaths on the floods.