Torrential rains and widespread flooding throughout southern Alberta on Thursday washed out roads and bridges, sent residents scurrying for safety and delivered surreal scenes of cars, couches and refrigerators just floating away.

"I woke up at about three o'clock in morning to the sound of this kind of rumbling and it was the creek," said Wade Graham, a resident of the mountain town of Canmore, west of Calgary.

"At first it was just intense, pretty powerful, amazing thing to watch. As daylight came, it just got bigger and bigger and wider and wider, and it's still getting bigger and bigger and wider and wider.

"All you can hear is like boulders and trees. I watched a refrigerator go by, I watched a shed go by, I watched couches go by. It's insane."

The Bow River Basin was battered late Wednesday and into Thursday with up to 100 millimetres of rain, bringing river levels to the breaking point. Parts of Calgary along the Bow and Elbow rivers remained in the crosshairs with residents in six low-lying areas out of their homes.

There were flashpoints of chaos from Banff and Canmore and Crowsnest Pass in the Rockies, to Calgary and beyond in the north and south to Lethbridge.

The flooding was particularly destructive in communities just south of Calgary such as High River, Turner Valley and Black Diamond, where the Highwood River swept away two people.

"One female adult had been stranded on a trailer and also a second adult male had been stranded on a nearby flatbed," said Cam Heke of STARS air ambulance.

In High River, the water trapped residents in their cars and forced others to flee to the rooftops of their homes.

Streets became tributaries, swamping vehicles.

Randy Livie said he came into town to help a friend and almost didn't get out.

"It was over my hood," he said. "There was a jeep in front of me and he stalled out.

"There was a minivan that went in front of me. He stalled out. This other car he came in and he started floating away - he bailed out. He had crutches. A truck pulled up and helped him out. It's just wild down there."

The river carried boats and trees into bridge abutments, he said.

High River Mounties were asking that people with motorboats help rescue at least a dozen stranded homeowners.

The province reported that 12 communities were under states of emergency.

Environment Canada issued a rainfall warning for the affected areas, estimating as much as 100 millimetres more rain could fall in the next two days.

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The Alberta Energy Regulator reported flooding may have caused a sour gas leak near Turner Valley. The flow of the toxic gas was turned off, but late Thursday a small amount was still seeping into flood waters submerging the line.

In the mountain parks, mudslides forced the closure of the Trans-Canada Highway, isolating Banff and Canmore.