Officials with the City of Calgary say as many as 100,000 people could be forced from their homes due to heavy flooding.
They say the evacuation would take place in stages over the next few days.
Residents of six low-lying areas are already out of their homes.
Torrential rains have created havoc in southern Alberta 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.