Flooding disrupted emergency telephone service across a broad swath of eastern Montana on Monday as parts of the state remained inundated and downstream communities prepared for the worst.
Residents of one southeast South Dakota town were told to be ready to leave their homes by Thursday as the Missouri River continued to rise.
Authorities said northwestern Montana and the Dakotas are next in line for high-water problems as mountain snow melts and record flows are released from the river's swollen reservoirs.
"This could be a pretty significant week for flooding, and it might expand," said Monique Lay, spokeswoman for the Montana Emergency Coordination Center.
Residents of Dakota Dunes, S.D., were told on Monday to be prepared to leave their homes by Thursday as the Missouri River continued to rise. Governor Dennis Daugaard stressed that no evacuation had been ordered, but the town's 2,500 people were told to prepare for the worst.
Eric Stasch, operations manager at Oahe Dam, located upstream in north-central South Dakota, said many levees being built in the Pierre-Fort Pierre area to protect public infrastructure also will shield private property. However, he said residents in flood-threatened areas should not assume the levees will be done in time or hold against the record dam releases.
Another 200 South Dakota National Guard soldiers were called in to help battle flooding along the river, bringing the total to more than 600 soldiers. Soldiers also have been summoned in parts of Montana and Wyoming.
Meanwhile, police officers went door-to-door in the Bismarck, N.D., neighbourhood of Southport telling residents to prepare to leave their homes because officials expect the area to eventually be flooded, making vehicle traffic impossible.
No evacuation was ordered, and city spokeswoman Gloria David said there was no timeline for a decision.
In Montana, flooding near Hardin on Sunday brought down telephone equipment that handles 911 and long-distance calls for Glendive, Miles City, Sidney, Fairview, Colstrip, Forsyth, Wibaux and Terry.
Emergency calls were rerouted until full service was restored at about 11 a.m. on Monday, said Michelle Jackson, a spokeswoman for phone service provider Qwest Communications.
In the central Montana town of Roundup, houses and businesses remained under several feet of water from the Musselshell River, which first topped its banks and swept through the town last Thursday. A mandatory boil-water order was issued, said Major Lori Hampa-Chamberlin of the Montana National Guard. Crews were working to pull stranded vehicles out of flooded areas and most routes into or out of the town were cut off.
"They're in trouble here," Ms. Hampa-Chamberlin said. "It's been raining nonstop."
The Musselshell River level had fallen about six inches by Monday. However, that drop was expected to slow with Monday's rain, said Brian Tesar of the U.S. National Weather Service in Billings.
Mr. Tesar said the expected warm-up later in the week raised the chances of flooding along the Yellowstone, Tongue and Shields rivers in central and eastern Montana.