At least three people were killed by floodwaters that devastated much of southern Alberta, leading authorities to evacuate the western Canadian city of Calgary's entire downtown.

Inside the city's hockey arena, the waters reached as high as the 10th row.

Canadian Prime Minister Stephen Harper called on Friday the level of flooding "stunning" and said officials don't know yet if it will get worse, but said the water has peaked and stabilised and noted that the weather has improved.

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Overflowing rivers washed out roads and bridges, soaked homes and turned streets into dirt-brown waterways around southern Alberta. Royal Canadian Mounted Police Sergeant Patricia Neely said three were dead and two bodies were recovered. The two bodies recovered are the two men who had been seen floating lifeless in the Highwood River near High River on Thursday, she said.

Mr Harper, a Calgary resident, said he never imagined there would be a flood of this magnitude in this part of Canada.

"This is incredible. I've seen a little bit of flooding in Calgary before. I don't think any of us have seen anything like this before. The magnitude is just extraordinary," he said.

"We're all very concerned that if gets much more than this it could have real impact on infrastructure and other services longer term, so we're hoping things will subside a bit."

Calgary Mayor Naheed Nenshi said the water levels have reached a peak, but have not declined.

"We've sat at the same level for many, many hours now," Mr Nenshi said. "There is one scenario that would it go even higher than this, so you'll either see the Bow river continue at this level for many hours or you will see it grow even higher and we're prepared for that eventuality."

Twenty-five neighborhoods in the city, with an estimated 75,000 residents, were evacuated due to floodwaters in Calgary, a city of more than a million people that hosted the 1988 Winter Olympics and is the center of Canada's oil industry.

Alberta Premier Alison Redford said Medicine Hat, east of Calgary, was under a mandatory evacuation order affecting 10,000 residents. The premier warned that communities downstream of Calgary had not yet felt the full force of the floodwaters.

About 350,000 people work in downtown Calgary on a typical day. However, officials said very few people need to be moved out, since many heeded warnings and did not go to work yesterday.

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A spokesman for Canada's defence minister said 1,300 soldiers from a base in Edmonton were being deployed to the flood zone.

Police were asking residents who were forced to leave the nearby High River area to register at evacuation shelter. The Town of High River remained under a mandatory evacuation order.

In downtown Calgary, water was inundating homes and businesses in the shadow of skyscrapers. Water has swamped cars and train tracks.

The city said the home rink of the National Hockey League's Calgary Flames flooded and the water inside was 10 rows deep. That would mean the dressing rooms are likely submerged as well.

"I think that really paints a very clear picture of what kinds of volumes of water we are dealing with," said Trevor Daroux, the city's deputy police chief.

About 1,500 have gone to emergency shelters while the rest have found shelter with family or friends, Mr Nenshi said.

The flood was forcing emergency plans at the Calgary Zoo, which is situated on an island near where the Elbow and Bow rivers meet. Lions and tigers were being prepared for transfer, if necessary, to prisoner holding cells at the courthouse.

Schools and court trials were cancelled on Friday and residents urged to avoid downtown. Transit service in the core was shut down.

Residents were left to wander and wade through streets waist-deep in water.

Newlyweds Scott and Marilyn Crowson were ordered out of their central Calgary condominium late last night as rising waters filled their parking garage and ruptured a nearby gas line. "That's just one building but every building is like this," he said. "For the most part, people are taking it in stride."

Crowson, a kayaker, estimated the Bow River, usually about four feet deep, is running at a depth of 15 feet.

"It's moving very, very fast," he said of the normally placid stream spanned by now-closed bridges. "I've never seen it so big and so high."

It had been a rainy week throughout much of Alberta, but on Thursday the Bow River Basin was battered with up to four inches of rain. Environment Canada's forecast called for more rain in the area, but in much smaller amounts.

Calgary was not alone in its weather-related woes. Flashpoints of chaos spread from towns in the Rockies south to Lethbridge.