Mayor Naheed Nenshi said the Elbow River, one of two rivers that flow through the southern Alberta city, has peaked.
And if things don't change, officials expect that the flow on the Bow River, which, in his words, looks like "an ocean at the moment", will remain steady for the next 12 hours.
No deaths have been reported, but many roads and underpasses have been washed. In the downtown, water is lapping at the doors of the Saddledome, home to the National Hockey League's Calgary Flames, and inundating homes and businesses in the shadow of skyscrapers. Water has swamped cars and train tracks
Nenshi said earlier he's never seen the rivers that high or that fast.
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Police urged people to stay away from downtown and not go to work.
Officials said lions and tigers from the Calgary Zoo may need to be transferred to prisoner holding cells at the downtown courthouse.
Alberta Premier Alison Redford promised the province will help flood victims put their lives back together and provide financial aid to communities that need to rebuild The premier said at a briefing that she has spoken to Prime Minister Stephen Harper, who is heading to Calgary and has promised disaster relief. She urged people to heed evacuation orders, so authorities could do their jobs. She called the flooding that has hit most of southern Alberta an "absolutely tragic situation.
The premier warned that communities downstream of Calgary have not yet felt the full force of the floodwaters. It had been a rainy week throughout much of Alberta, but yesterday the Bow River Basin was battered with up to 100 millimetres of rain. Environment Canada's forecast calls for more rain in the area, but in much smaller amounts.