ROCK PORT, Mo. -- A partial breach has appeared on a levee along the Missouri River in northwest Missouri, the U.S. Army Corps of Engineers said Monday, a day after officials ordered hundreds of residents in a southwest Iowa town to evacuate their homes in response to another hole in the earthen barrier.

The Iowa National Guard has dropped thousands of pounds of sandbags to help support the levee, but the corps expects the levee to fully breach as water levels rise. The corps has predicted record flows along the Missouri River and large releases from several upstream reservoirs because of steady spring rain and runoff from record snowpack.

The corps reported the first partial breach in the levee in Atchison County, Mo. -- a hole 1 inch to 1 1/2 inches in diameter -- on Sunday. Atchison County officials said the first break did not constitute a full breach because of its size, and called it instead, a "compromise." The second partial breach in the levee, near the Iowa border, is about 10 to 15 feet wide, the corps said Monday.

Authorities ordered 600 residents of Hamburg, Iowa -- nearly half the town -- to evacuate their homes on Sunday after the first breach was discovered, said Stefanie Bond, spokeswoman for the Iowa Department of Homeland Security and Emergency Management. The residents, most of them on the south side of the city of 1,141, were told to get out within 24 hours.

Several people living in Atchison County have also been ordered to leave.

"People's safety is our No. 1 concern, so we want to stress how important it is for the public to stay off of these levees as we continue to assess the risk," said the corps' Omaha District Commander Col. Bob Ruch.

"We acknowledge the frustrations of the affected communities, and we are committed to working together to avoid the loss of life and minimize damages," said Ruch.

He said the corps has been working to raise the levee near Hamburg an additional 5 feet to help protect the town.

Officials are also concerned about a section of a levee near Brownville, Neb., and crews are trying to determine the extent of possible damage there, the corps said in a release Monday.

Gen. Derek Hill, head of the Iowa Department of Homeland Security and Emergency Management, characterized the first breach as a "boil" -- a leak that "shoots out like a small geyser."

Iowa sent a Blackhawk helicopter Sunday to drop 1,000-pound sandbags on the levee, Hill said, adding it was too dangerous to use ground crews. It was not known how long the work would take.

"It's a technique that's been used before by the corps," he said. "There's no guarantees but we hope it will (work)."

Rhonda Wiley, emergency management director for Atchison County, Mo., said Sunday that levees along the Missouri River have been weakened by the river's recent high water levels.

"We anticipate these compromises rearing their ugly heads all up and down the levee system throughout this event," Wiley said Sunday. "It's not a pretty picture. But today nobody appears to be in imminent danger at this moment."

In South Dakota, the Army Corps of Engineers began construction of a backup levee Sunday to protect the town of Dakota Dunes. Corps engineer LeeJay Templeton said the 1.4-mile long secondary levee is slated to be completed by Thursday.

The Missouri River was expected to rise about 8 feet to 1,098 feet above sea level by June 14 in the city of about 2,500 people, some of whom have evacuated ahead of the planned crest. Officials said construction of the primary levee is still under way to protect the city 2 feet beyond the projected high level.