There were no confirmed fatalities or injuries, but there were unverified reports that between two to four people had been swept away by onrushing torrents.
Thousands of police, military personnel, government workers and ordinary citizens were involved in rescue operations in a vast area of the foothills of the Rockies.
Floodwaters submerge cars in High River, Alberta, on Thursday.
The city of Calgary, home to almost 1.1 million people, was badly affected along with the communities of Canmore, Bragg Creek and High River.
Calgary city officials declared a state of local emergency and more than 75,000 Calgarians – about 7 percent of city’s residents – were ordered to evacuate and take shelter with friends and relatives or in leisure centers, Reuters and other media reported.
Thousands of others in outlying areas were also told to get out, with officials telling The Associated Press that as many as 100,000 people could be forced from their homes.
Calgary city mayor Naheed Nenshi, said at a press conference at 5 a.m. local time (7 a.m. ET) Friday that no one in the city should travel unless it was “absolutely necessary” with the Bow and Elbow rivers both causing floods.
“The Bow is certainly moving higher and faster than I have ever seen in my lifetime,” Nenshi explained, adding that the Elbow appeared to have reached its peak but could stay there for some time.
“I saw water levels that were incredibly high. I saw areas of the city that were deeply under water, things I’ve never seen before.”
He added that “knock on wood” no-one had been injured and killed and praised police and others involved in flood-related work as “heroes.”
Chief Superintendent Kevin Harrison, of the Royal Canadian Mounted Police, said the area affected by flooding was immense.
Water approaches homes near Cougar Creek in Canmore, Alberta, on Thursday.
“This is widespread … just guessing, the area that is flooded would be bigger than England,” he said. England is about the same size as New York state.
“The magnitude of it is certainly overwhelming.”
He said large parts of Calgary and most of southern Alberta’s 420,000 residents were likely affected by the floods in some way.
About 150 people were rescued by helicopters in High River alone as they stood on rooftops, clung to trees or from inside vehicles, Harrison said.
He said heavy rain saw flooding begin at about 1:30 a.m. local time (3:30 a.m. ET) Thursday.
Harrison said it had stopped raining at 5:30 a.m. local time (7:30 a.m. ET), but more was expected.
He said RCMP were investigating reports from rescuers that two or three men were seen being washed away in the floods.
A woman who had been stranded on top of a trailer was missing after it was swept away, STARS air ambulance spokesman Cam Heke told the AP.
Environment Canada issued rainfall alerts early Friday for parts of the state.
“Some areas have already received over 100 millimetres [4 inches] of rain since Wednesday night,” it said. “The heaviest rain is expected in regions north and west of Calgary today where an additional 10 to 25 millimetres are forecast to fall. The rain should continue to taper off tonight and into Saturday.”
Some rivers were not expected to crest in some parts of the state until Saturday.
Wade Graham, a resident of Canmore, told the AP on Wednesday that he woke early to a rumbling sound coming from a nearby 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,” he said.
He added: “I watched a refrigerator go by, I watched a shed go by, I watched couches go by. It’s insane.”
The Associated Press and Reuters contributed to this report.