South Dakota Gov. Dennis Daugaard says work is moving quickly on levees being built to protect the small southeastern town of Dakota Dunes from the rising Missouri River.
He says people in the city still should move their possessions to higher ground and prepare to evacuate later in the week.
The U.S. Army Corps of Engineers is releasing record amounts of water from dams on the Missouri River because of runoff from heavy spring snowmelt and rains.
Levees also are being built in the state capital of Pierre and nearby Fort Pierre, where water is expected to rise four feet within a week.
Officials say the Dakota Dunes levees should protect two feet higher than rivers crest, but evacuation may be necessary in case the levees don't hold.
The Missouri River is rising downstream of communities already flooded by the record-breaking rain and heavy snowmelt.
The operations manager at a dam in north-central South Dakota says residents could be away from their homes for as long as two months.
High waters also have affected Montana, disrupting emergency phone service in the east.
Rains are adding more water to rivers and streams that had started to recede over the weekend, and snow that's falling in the mountains is expected to melt later in the week, triggering another round of floods.
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