A historic snowy winter is turning into record spring flooding across a wide area in the middle of the United States, as major rivers spill over their banks, break levees and inundate towns and farms. The governors of Nebraska, South Dakota, and Wisconsin have declared emergencies, and Iowa’s governor has issued a disaster proclamation. At least two people in Nebraska have died in the floodwaters, and two others are missing.

Hundreds of families have fled their homes, especially in the Mississippi and Missouri flood plains, where levees were breached in many areas. Offutt Air Force Base, outside Omaha, said that one-third of the base was underwater on Sunday. Even the National Weather Service said it had to evacuate its offices in Omaha on Friday because of rising water.

[Read more on how the record floods have left a devastating toll on farmers and ranchers.]

What touched off the flooding?

Rain was the immediate cause. The Weather Service’s Omaha office recorded 1.37 inches last week; more fell to the north and west, with Norfolk, Neb., getting 2.27 inches Tuesday, Wednesday and Thursday.