Heavy rain combined with water from the snowpack has saturated the ground and driven the Missouri and its tributaries over their banks from Montana to Missouri.
To manage the flooding, the U.S. Army Corps of Engineers is releasing at least 130,000 cubic feet of water per second from each of five flood-control dams, and about 50,700 cubic feet per second from a sixth.
The largest release is at the Oahe Dam just north of Pierre, S.D., where 150,700 cubic feet per second is being released - 24 times the amount of water flowing through the Potomac River at Harpers Ferry, W. Va.
"There's been nearly a year's worth of rainfall in the last few weeks," said Lynn Maximuk, the weather service's central region director in Kansas City.
In Iowa, 100 homes were evacuated in Council Bluffs and 300 other people were forced to flee earlier this week, according to a statement from the state's joint information center.
Iowa Gov. Terry E. Branstad issued an emergency declaration for six counties along the river. Parts of I-29 and I-680 in Iowa have been closed.
On the west bank of the river in Nebraska, the Fort Calhoun Nuclear Power Plant, which shut for refueling April 9, has remained closed because of the flooding.
Last month, 130 oil wells, representing 1 percent of North Dakota's output, were idled by flooding along the Missouri.