PIERRE, S.D. — Crews raced approaching floodwaters Tuesday to complete emergency levees aimed at protecting South Dakota’s capital city and two other towns as the swollen Missouri River rolled downstream from the Northern Plains. Meanwhile, the mayor of Minot, N.D., ordered a quarter of the city’s residents to evacuate areas along the flooding Souris River.

Residents of the upscale community of Dakota Dunes in southeastern South Dakota, below the final dam on the river, have been told to move their possessions to higher ground and be ready to leave their homes by Thursday, a day before releases from the dams are set to increase again.

Several thousand people in Pierre, the state capital, and neighboring Fort Pierre on the west bank have been working day and night since late last week to lay sandbags around their homes and move to safety.

Those forced to leave their homes may not be able to return for two months or more. No evacuation orders had been issued Tuesday in South Dakota, but many people in the three cities had already moved to safer places.

“We’re going to fight this flood with every fiber of our beings, and we’ll do everything we can to minimize its effects,” Gov. Dennis Daugaard said.

In Minot, N.D., Mayor Curt Zimbelman said the evacuation order affects about 10,000 residents who live along a four-mile stretch of the Souris, which has risen with rain, snowmelt and discharges from Lake Darling. Ziwmbelman said residents are expected be out of their homes by Wednesday night, in part to give construction crews room to raise and reinforce earthen dikes in the area. The Souris is part of a different river system than the Missouri.

The U.S. Army Corps of Engineers is increasing releases from the six dams on the Missouri River to drain water from record rains of up to 8 inches that fell in eastern Montana and Wyoming and western North Dakota and South Dakota in the past two weeks. Heavy runoff from melting snow in the northern Rocky Mountains is expected to add to the problem soon.

Flooding in Montana has damaged at least 200 homes on the Crow Indian Reservation and many more homes and businesses in other areas. Water levels had dropped more than 2 feet in the central Montana town of Roundup, but warmer weather later in the week is expected to cause a new round of flooding in parts of the state as mountain snows melt.

In North Dakota, more than seven miles of levees were being built in Bismarck and another three and a half miles were going up across the river in Mandan.

Officials in western Iowa, downstream from Dakota Dunes, were making plans to deal with flooding in Sioux City and other areas. The Nebraska towns of Niobara and Santee are already dealing with flooding from the Lewis and Clark Reservoir, while cities further downstream are preparing for high water over the next month.

In the three South Dakota cities, streets were busy with National Guard trucks, pickups carrying sandbags and large trucks carrying clay to build the levees. Many homes had already been surrounded with walls of sandbags that were up to 6-feet high.

Daugaard said the earthen levees were being built to 2 feet above the expected crest in all three towns, but he urged residents not to count on the levees to protect them.

“Citizens should assume the worst, that we will be unsuccessful in getting the levees raised in time or that the levees, once raised, will not hold,” the governor said.

He said no deaths had been reported from flooding, and for now, the concern is about saving property.