Mayor Mary Lee Nielson told businesses to stay closed for at least this week, unless they are absolutely needed.
Nielson also had asked residents on Friday to leave the city of about 7,000 after river water overwhelmed the sewer system.
But Nielson said Monday she was seeing more traffic, and the North Dakota National Guard had said late Sunday that only 423 homes in Valley City were evacuated.
"Obviously, I'm worried about that," Nielson said. People may not understand the consequences of putting more pressure on the city's fragile roads, bridges and services, she said.
In addition, high water has closed seven of the town's eight street bridges, reducing the routes available for emergency vehicles, she said.
Nielson said the town's flood-damaged sewer system was still not usable, although the town had begun pumping sewage into a holding lagoon rather than directly into the river.
"We'll be using portable potties, probably for another month," the mayor said.
The Army Corps of Engineers said it had reduced the amount of water released from the Baldhill Dam upstream.
Corps spokesman Mark Davidson says hydrologists believe the Sheyenne has crested at towns above the dam.
Southwest of Valley City in LaMoure County, the National Guard placed more than 290 one-ton sandbags in an effort to slow erosion of an emergency spillway at a dam along Cottonwood Creek. Authorities said it is about 20 miles from the nearest community and the greatest flood threat is to farms and roads.
National Guard Lt. Col. Rick Smith said Monday that the spillway was eroding so quickly that the Guard had ordered up 10 commercial dump trucks full of rock to build a secondary dike.
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