The mayor of a flood-threatened North Dakota town asked all residents to leave on Friday after the sewer system backed up, flooding offices with murky water and making tap water unsafe to drink.

Mayor Mary Lee Nielson urged those of the town's 7,000 residents who had not already cleared out to leave. She said tap water should be avoided because it could be tainted.

Valley City is threatened by the swollen Sheyenne River, which topped out on Monday at 20 feet high, a record in the area. It remained Friday at nearly 20.5 feet, and officials have warned that the danger of flooding could persist for weeks.

Spokeswoman Julie Nelson said the police station and the Valley City Times-Record's offices were among the places where sewage had backed up.

At the Barnes County Historical Society, curator Wes Anderson scrambled to plug toilets and control the damage.

"I heard this gurgle, gurgle, gurgle in the upstairs sink and I thought, 'This isn't good,"' Anderson said. "Then I heard something in the downstairs sink and I thought, 'This is worse.'

"We've got about an inch or more of brown water floating through here," he said. He already had moved valuable papers to high ground, he said. But antique farm equipment and other large items were too heavy to move.

Plumbers came to help control the damage, Anderson said. By late morning, the backup was down to a trickle. "If we hadn't capped it off, it would have gone to the ceiling," he said.

Ruth Venhuizen, who had been staying with a co-worker since leaving her home earlier in the week, said the new evacuation order meant she'd have to spend the night at a Red Cross shelter in nearby Oriska.

"It's very sad, but I have no place else to go," Venhuizen said, fighting back tears. "I hope it's not for long."

The news was better Friday in Fargo, about 60 miles east. Mayor Dennis Walaker said he thinks the Red River has crested for a second time, though he said the flight fight is not over. The river is at just under 34 feet Friday, 7 feet lower than its first crest last month at a record of nearly 41 feet.

In tiny Kathryn, N.D., the National Guard said sandbags placed along a leaky dam to shore it up were holding Friday morning. The town's 55 residents evacuated Wednesday because floodwater was undermining the nearby dam.