(CNN) -- The swollen Missouri River washed over and punched through levees in northwestern Missouri early Sunday, spurring authorities to urge about 250 nearby residents to leave their homes.

The river breached levees at two points overnight and overtopped them at two others near Corning, Missouri, about 100 miles north of Kansas City, the Holt County Sheriff's Department reported. Most residents had already cleared out due to voluntary evacuation calls, but authorities went door-to-door early Sunday to order nearly 30 people who remained to leave, the agency told CNN.

Upriver, evacuation advisories went out for 200 to 250 people in the town of Watson and other areas west of Interstate 29, said Mark Manchester, the deputy emergency management director in Atchison County. Water was sloshing over the levees "at a pretty good clip" Sunday morning and had already topped the county's previous record mark, set in 1993, he said.

"We're in uncharted waters here," Manchester said.

The Missouri was more than 11.5 feet over flood stage at nearby Brownville, according to National Weather Service data.

And across the state line in Hamburg, Iowa, where two levees failed last week, the river was expected to crest at 10 feet over flood stage in the coming days, Fremont County Emergency Management chief Mike Crecelius said.

Crecelius said the river has been over flood stage since late April, and that forecasters are projecting river flows of 150,000 cubic feet (1.1 million gallons) per second through August.

"They're not designed for this amount of pressure for this length of time," Crecelius said. "They've never been tested like this."

Heavy rainfall in Montana and North Dakota, combined with melting snow from the Rocky Mountains, have sent the Missouri surging downstream, according to the National Weather Service. The 6 to 12 inches of rainfall in the upper Missouri basin in the past few weeks is nearly a normal year's worth, and runoff from the mountain snowpack is 140% of normal, the agency says.

CNN's Matt Smith and Divina Mims contributed to this report.