The floodwaters have washed out roads and bridges, left at least one person missing and caused cars, couches and refrigerators to float away. Several communities are under states of emergency, and as many as 100,000 people could be forced from their homes in Calgary alone.
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.
John E. Marriott, a photographer who lives along Cougar Creek in the mountain town of Canmore, about 100 km west of Calgary, said on Thursday that the raging torrent had moved within metres of his house.
The creek rose from centimetres deep and about three metres wide to being 150 metres wide and about five metres deep, he said.
Canmore resident Wade Graham said he woke up at about 3 a.m. Thursday to a rumbling sound "and it was the creek."
"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."
He told CBC News he was looking directly at Grotto Mountain, "which has no waterfalls on it ever. From my bedroom window I can count ... seven major waterfalls from here."
Mike Crawford, who lives on Bow Crescent in Calgary, said he had to leave his home just after noon Thursday.
"Early this morning the river was going over the bike trails, but it wasn't looking to be a concern," he told CBC's Ian Hanomansing.
"Noon it was higher, but still not a concern, and within hours it had risen several feet and water started seeping up onto the streets and running through the neighbour's yard into my yard."
He said he decided to leave while it was still safe, saying water was starting to cover the streets. Crawford's home was built over the last year, and he moved in only three weeks ago. "I'm not really sure what I'm going to be walking into tomorrow or the next day," he said.
The community of Canmore, a popular tourist destination, also had flood trouble. Writer and musician Dave Bidini was in Canmore working on a book, and is now stranded in a hotel with truckers, tourists and families with children.
"There’s no way in and there’s no way out [of town]," he told CBC News shortly before midnight local time. "I know people at the hotel were worrying about looking a few days down the road, about supplies, and the town really kind of in a state of atrophy, which is where it appears to be headed right now. So it’s actually starting to get kind of scary.”
He described the hotel as "lively," with people on edge and worried about power outages.
“Nobody by any means is relaxed and I do think it’s going to be kind of a long night," he said.
Evacuees asked to mark doors with X
Bruce Burrell, director of the Calgary Emergency Management Agency, said water levels on the Bow River (Cougar Creek is a tributary) aren't expected to subside until Saturday afternoon.
Evacuees in Calgary were being asked to stay with friends or relatives, though recreation centres were being set up to accommodate those who had no place to go.
They were also asked to mark their front doors with a giant "X" so that first responders wouldn't have to stop at homes that were already emptied.
Calgary resident Rick Wasfy said he was told to leave by four police officers even though it was sunny and the water was seven blocks from his home.
Wasfy, who was forced to ride out Hurricane Katrina in 2005, said packing and heading to a friend's house was the right choice.
"Mother Nature’s not something you want to mess with. If you have a chance to get out and get safe, that’s the right thing to do," he said.
His advice to people who decide to stay despite the evacuations was that, "The water's going to come suddenly when it comes."
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."
Trapped in their cars
In High River, where residents were under a mandatory evacuation order by late Thursday, 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.
Megan Wozniuk and her half-sister, who is eight and a half months pregnant, paddled in a boat for a "frightening" 45-minute journey to rescue her half-sister's father. But when they got there, he didn't want to leave.
"It's about 10 hours since we've heard anything" from him, she told CBC News.
'Save the people'
High River spokesperson Joan Botkin said Thursday that 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."
- List of Alberta emergency numbers
- Read about how floods happen and what areas are vulnerable
Alberta Health Services said the emergency department in High River had been closed, though patients already in the hospital were safe and being cared for.
Residents in a seniors care facility were told to leave. Even the original evacuation centre had to be moved as floodwaters threatened.
Cellphone service and landline service in the area was spotty, CBC's Briar Stewart reported, making the situation more difficult in the community of roughly 12,000. "People were really surprised at just how fast the water moved in," Stewart said. "We're told that at one point 150 people were stranded, many of them on their roof, trying to wave in the helicopters in the air to get their attention."