Western Montana's Clark Fork River reached major flood stage Thursday for only the sixth time since measurements have been taken, threatening additional homes after authorities previously ordered the evacuation of 60 houses and trailers near Missoula.

The Clark Fork surged past 13 feet (3.96 meters), and it's expected to hover around that level for at least a week, National Weather Service meteorologist Alex Lukinbeal said.

"I think a concerning aspect of this is the duration at which the river is going to be above moderate flood stage," Lukinbeal said. "The river has a long time to do damage."

At major flood stage, between 800 and 1,300 homes in low-lying areas across Missoula could be threatened, officials have said. About 820 homes were under an evacuation warning Thursday.

An evacuation order was issued Wednesday for 60 homes, but many residents defied the order to stay with their property.

"This will be a critical time for our team to assess if and when and where additional evacuation orders will be necessary," Sheriff T.J. McDermott said, according to the Missoulian.

Water pushed three mobile homes off their foundations, and authorities worried they could be swept away and collide with a bridge downstream.

"Yesterday we tried to go there to retrieve them, but it was too dangerous with the flooding and debris — there's full trees floating by them," Missoula sheriff's Capt. Anthony Rio said.

Missoula authorities rescued a man whose tent was surrounded by water in a homeless encampment near the river, KECI-TV reported. Authorities moved others staying at the encampment and were looking to place them in shelters, the station reported.

Forecasters expect the Clark Fork River to crest over the weekend at 13 ½ feet, which would be the third-highest level ever recorded. The record was set in 1908, when the river crested at more than 17 feet.

Nearly all of the water is coming from the rapidly melting snowpack in the mountains around Missoula. Near-record snowfall over the winter has caused flooding this spring across western Montana and parts of central Montana.

In central Montana, the Blackfoot River and surrounding streams covered roads and threatened about 100 homes in Lincoln. Some homes already have taken in water resident Shane Brown told The Great Falls Tribune.

"The waters are just coming up," Brown said. "We deal with it every year. This year, with all the snowfall, it's definitely going to get higher than usual."