BILLINGS, Mont. — More than 800 western Montana residences were put on notice that they could have to evacuate in coming days as rivers swollen by melting mountain snows flooded low-lying areas of Missoula on Sunday.

Officials braced for a potentially prolonged inundation that one official said could be the worst in more than 40 years.

The Clark Fork River near Missoula was expected to approach major flood stage as early as Tuesday and to crest Friday, according to the National Weather Service. It would be the river's highest level since 1975, said Ken Parks, deputy coordinator for Missoula County Disaster and Emergency Services.

An official evacuation order had not been issued but Parks said 830 residences had been identified for potential evacuation and warned the figure could grow.

"If you live anywhere near a stream or waterway in western Montana you need to be prepared to leave your home," he said. "This is going to come earlier than we expected. We're trying to get out ahead of this thing and get the message out that this could be a very dangerous situation."

The National Weather Service extended a flood warning for the Clark Fork above Missoula until further notice. High waters could linger for the next three or four weeks on the Clark Fork, the Bitterroot River, the Blackfoot River and areas around Seeley Lake, Parks said.

About 40 households in Missoula's Orchard Homes neighborhood already had experienced some flooding by Sunday afternoon, he said, receiving anywhere from a couple inches to several feet of water.

No injuries were reported.

County officials set up a sandbagging station at the Orchard Homes Country Life Club for residents who wanted to protect their properties.

Downstream, about 75 miles northwest of Missoula, erosion along the Clark Fork brought high waters dangerously close to holding ponds for the wastewater treatment plant in the town of Plains, KECI-TV reported. The U.S. Army Corps of Engineers was asked to help the town secure the ponds.

Widespread flooding was possible later in the week from East Missoula to the Bitterroot River, including impacts to U.S. Highway 10, forecasters said.

Temperatures were expected to reach the 70s, which could accelerate melting of a mountain snowpack that hit record depths in parts of the state this winter.

Thunderstorms in the forecast for coming days threatened to make matters worse. Up to three-quarters of an inch of rain could fall in some areas, according to the weather service.

"No matter what we're going to have a lot of water coming out of the mountains," Parks said. "We really can't win."

Snowmelt at high elevations also was expected to cause minor flooding in the Helena area, south of Wolf Creek, near Augusta and at locations around Lincoln including the Stemple Pass area.

To the north, near Browning and Glacier National Park, Badger Creek below the Four Horns Canal was expected to flood Sunday, although with only minor impacts. A flood advisory for the region was in effect until Monday night.

In eastern Montana, river levels continued to fall after cresting last week. A flood warning was cancelled for the Nashua area but remained in effect for Glasgow until Tuesday morning.

Gov. Steve Bullock on Wednesday declared a statewide flooding emergency. That allows the use of state resources and the Montana National Guard for flood protection.

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Follow Matthew Brown on Twitter at @matthewbrownap