Torrential rains and widespread flooding throughout southern Alberta on Thursday washed out roads and bridges, sent residents scurrying for safety, and delivered up surreal scenes of cars, couches and refrigerators just floating away.
The RCMP put out a call for help to the Canadian Armed Forces, which sent in two helicopters and a Hercules aircraft to help extract people stranded by water.
Officials with the City of Calgary said as many as 100,000 people in low-lying neighbourhoods could be forced from their homes due to heavy flooding, an evacuation that would take place in stages over the next few days.
Bruce Burrell, director of the Calgary Emergency Management Agency, said water levels on the Bow River aren't expected to subside until Saturday afternoon.
"Depending on the extent of flooding we experience overnight, there may be areas of the city where people are not going to be able to get into until the weekend," he told a news conference.
Burrell said they suggested that workers in downtown Calgary, which borders on the river, leave work early Thursday if they live in threatened areas, so they could prepare to evacuate.
Other workers were advised to consider staying late so as to lighten the rush-hour load on roads that might be needed for evacuations.
The Calgary Zoo, located on St. George's Island, shut down in the afternoon and said it would also be closed Friday.
"Rest assured Animal Care are working closely with our facilities team to ensure the safety of all of our animals during this period of flood risk and are following our established emergency protocols," the zoo said on its website.
The Bow River Basin was battered with up to 100 mm of rain.
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.
"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 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.
"We did respond to the area. The female adult was no longer on the trailer and was missing. We did conduct a search along the river and we were unable to locate that missing person.
"However, the male adult was on the trailer and local emergency services with another helicopter organization ... were able to rescue that man."
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.
"We have people on their rooftops who were unable to evacuate fast enough," said RCMP Sgt. Patricia Neely.
Town spokesperson Joan Botkin said the rescue boats were struggling with a strong current, but added, "that's our priority. Right now it is 'Save the people. Get the residents out of there."'
Danielle Smith, a Highwood resident and Wildrose Opposition leader in the legislature, said she and other residents frantically sandbagged around the hospital, but could not keep water from rushing past the doors, forcing patients to higher floors.
"I've been talking to people who have been here for 35 years or more who saw the 2005 flood, who saw the 1995 flood, and say this is way worse," Smith told CHQR radio station in Calgary.
Residents in a seniors care facility were told to leave. Even the original evacuation centre had to be moved as floodwaters threatened.
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.
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 floodwaters submerging the line. The Alberta Energy Regulator said public safety was not threatened.
In the mountain parks, mudslides forced the closure of the Trans-Canada Highway, isolating Banff and Canmore. Campers at Two Jack Lakeside campground were moved to higher ground.
Highways north and south of Banff were also shut down.
In Canmore, some homeowners saw the raging Cougar Creek eat away half their backyards, leaving behind crumbling wooden stairwells and twisted fences jutting out over torrents of water. Power and gas were out in some parts of town.
Parks Canada spokeswoman Michelle Macullo said people caught inside the park didn't really have many options.
"Right now, if people are in Canmore, they can get to Banff. People from Banff can get to Canmore," she said. "We just have to wait to see what the weather presents."
A mandatory evacuation order was in effect for Bragg Creek.
In Lethbridge, a few neighbourhoods were evacuated and city officials urged parents to pick up their children early from school. They expected the Oldman River would rise and cut off travel across the city.
In Crowsnest Pass and Okotoks, some residents were ordered out. In Sundre, 80 km northwest of Calgary, dozens of homes were under mandatory evacuation order along both sides of the swollen Red Deer River.
Municipal Affairs Minister Doug Griffiths said the province was responding to requests as they came in.
"The municipalities have asked for no extra equipment or resources except in relation to some evacuations, which we've managed to acquire a few helicopters to help with those isolated evacuations," he said.
Premier Alison Redford was returning home to Alberta from a conference in New York.
"I plan to visit the affected areas as soon as possible on Friday to see the situation firsthand and to thank those who have been working so hard at keeping everyone safe," she said in a news release.
— With files from Jennifer Graham