Communities south of Calgary are the worst affected so far, with water levels in massively swollen creeks not expected to drop until Saturday afternoon.
Canmore resident Wade Graham said he had watched as the foundations of a row of houses was washed away.
An estimated 230,000 people live and work in central Calgary, officials believed there would be few people to evacuate because many did not come to work on Friday.
Some 25 neighbourhoods in Calgary, a city of one million, had already been evacuated. An estimated 75,000 residents have been ordered out of their homes.
Heavy rains eight years ago caused flood damage to about 40,000 Calgary homes and resulted in the evacuation of more than 1,500 Calgarians. It resulted in $275 million in insured losses.
There was water in the streets of the Bowness area in the city’s northwest. The city has not said to what extent any homes have been flooded.
Entire neighbourhoods all along the two rivers have been cleared of inhabitants. City officials have estimated that as many as 100,000 could be out of their homes.
As many as four people may have been killed in Alberta, police said. The bodies of two men were seen floating down a river and a woman washed away with her campervan was missing. Another man was seen falling from a canoe in the Highwood river and it was not known whether he had reached safety.
In downtown Calgary water inundated homes and businesses in the shadow of skyscrapers. Water swamped cars and train tracks.
Some of the worst flooding hit High River, where it was estimated half of the people in the town had their homes flooded.
Prime Minister Stephen Harper’s full statement:
Our thoughts and prayers are with those families who have been affected by the serious flooding in Calgary and Southern Alberta.
I want to assure Albertans that the federal government has offered any and all possible assistance to the Province of Alberta in response to the situation. Canadian Armed Forces assets, including a Cormorant and Griffon helicopter, have been deployed to the area to assist the Alberta Provincial Emergency team with rescue and evacuation efforts.
We hope for a speedy end to the flooding and return to safe conditions as soon as possible. We remain ready to provide additional assistance if requested by provincial authorities.