MINOT, N.D. (AP) — Officials in North Dakota’s fourth-largest city said Thursday they had done all they could to protect critical infrastructure from the rising Souris River as it headed toward a record flood.

Minot Mayor Curt Zimbelman said dikes were raised as high as possible around the city’s sewer lift station and couldn’t be raised any higher.

“We need to hope that they hold,” Zimbelman said. The city was confident the water plant was protected.

A failure of those protections would worsen a desperate situation in Minot, where as many as 10,000 people — about a fourth of the city’s population — were ordered to evacuate Wednesday.

The city slightly expanded the evacuation zone on Thursday to add about 400 people in the river valley, but the notice was voluntary. Several hours after the expanded zone was announced, officials said damage to those homes might be no more than water in basements.

Swollen by heavy rains and snowmelt far upstream, the Souris has risen rapidly since the weekend. On Thursday, officials accelerated the release of water from the Lake Darling dam and said it could raise the river 2 to 3 feet higher than earlier projections — to as much as 6 1/2 feet above the record set in 1881. The peak was expected sometime Saturday or Sunday.

In Burlington, a town of about 1,000 people a few miles upstream on the confluence of the Souris and Des Lacs rivers, city officials abandoned sandbagging as hopeless and sent people to Minot to help out. About a third of the town’s 320 houses are expected to be lost.

“We’re no longer able to save the city,” Burlington Mayor Jerome Gruenberg said. Founded in 1883, Burlington is the oldest town in Ward County.

Thursday was a day of frenzied labor around Minot, a town best known for its Air Force base but also an important agricultural center and home to many laborers drawn to the oil boom in western North Dakota.

Heavy equipment hauled dirt and clay to raise dikes wherever possible — an effort Zimbelman said would continue until rising water made it impossible. Workers and National Guard members were the only people to be seen in evacuated areas.

Fast-flowing water had overtopped dikes in some places and risen to the first floor on several homes. A trailer park was under water. In one area, an old Chevy was half-submerged.

Near the water treatment plant, water had risen above a bridge deck; orange barricades blocked any traffic at either end. Loose clothes and dark trash bags could be seen floating in the Souris, cast off by departing residents.

Broadway Bridge, on a major north-south artery, was closed around midday and officials fretted over the possible closure of other bridges that would effectively cut the city in two. Two bridges remained open.

Kathy Sivertson, 52, who lives a block outside the initial evacuation zone, was opting to ignore the recommendation for expanded evacuations. She spent part of Thursday moving her belongings out of her basement but said she’d stay in her house until “they kick me out.”

Meanwhile, Leon Delker, 55, who lives nine blocks from the river, brought in a survey crew that estimated the water would go 3 feet up on his front door. He planned to clear out everything but the American flag in front of his home and “stay out until this thing is over.”

Before the Broadway Bridge closed, many people were using it as a sightseeing perch — some to check on their own homes.

Jodine Blake, 45, watched as water approached her two-story house, which stood out among others with its orange paint. She had moved some belongings to the second story in the hope they would be safe there.

“It just makes you cry. You lose everything,” she said.

Dan Vander Vorste, 55, helped residents in two of his rental houses move out of the evacuation zone. He said he went through the historic flood of 1969 — which was eclipsed Thursday — and knows what lies ahead.

“It’s going to be five days of shock followed by reality,” he said.