Fort Pierre, S.D. - Sitting atop a 6-foot wall of white sandbags hastily stacked to protect his home from the rising Missouri River, 82-year-old Helmet Reuer doesn't buy the official explanation that heavy rains caused a sudden flood threat.

Along with his neighbors in an upscale section of Fort Pierre, Reuer thinks the U.S. Army Corps of Engineers blew it, waiting until too late to begin releasing water through the Missouri's six dams to give itself a cushion against potential flooding.

"It's human error," Reuer said as rising water neared his trim gray house.

Corps officials insist otherwise. They say they were in good shape to handle spring rain and melt from a massive Rocky Mountain snowpack until unexpectedly heavy rains of 8 inches or more fell last month in eastern Montana and Wyoming and western North Dakota and South Dakota.

"This is just a massive rain that fell in the exact wrong place at the exact wrong time," said Eric Stasch, operations manager at Oahe Dam, the huge structure that controls the Missouri's flow just above Fort Pierre and nearby Pierre, South Dakota's capital.

Crews have worked urgently all week to build up levee protections for the two cities, and say they expect to have 2 feet to spare. But Gov. Dennis Daugaard advised people in neighborhoods nearest the river to leave voluntarily in case levees don't hold, and hundreds have done so after a hectic week of moving possessions and adding sandbags around their houses.

They face weeks out of their homes until the river begins cresting in mid-June, with high water expected to linger for up to two months. The small town of Dakota Dunes, S.D., in the southeastern tip of the state, has also erected levees, as has Bismarck, N.D., though the situation is less serious there.

"I think they screwed up royally," former Gov. Mike Rounds said of the Corps, as he moved some possessions from the riverbank house he and his wife built and moved into after he left office in January. "I think they forgot their No. 1 mission, and that's flood protection."

People here were prepared for some higher flows, but many were startled when the Corps announced May 26 it needed to release water much faster than expected from the dams in Montana and the Dakotas.

Jody Farhat, chief of Missouri River Basin water management in the corps' Omaha District, said the agency made no mistakes and has managed releases in accordance with its manual. She said conditions on May 1 indicated peak releases at only a third of what they're now projected, and the reservoir system had full capacity to deal with flood control at the start of the runoff season. All that changed with the record rainfall in the upper basin and additional snow in the mountains, she said.

Farhat said heavy runoff from last year was released before the start of this year's runoff season, and discharges this spring were above normal even before the heavy rainfall upstream.