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.
The bodies of two men were found near High River, Alberta, a town of 13,000 located about 60 km (37 miles) south of Calgary. The Royal Canadian Mounted Police said two other people were missing, including a woman who was swept away with her camper.
Some 1,300 troops were deployed to help with rescues and the mandatory evacuations that forced 100,000 people from their homes in Calgary and thousands more in the small towns surrounding the city.
Canadian Prime Minister Stephen Harper toured the affected area with local authorities on Friday by helicopter and said his government "is taking swift action."
"Members of the RCMP and the Canadian Armed Forces are already on the ground helping with search and rescue, evacuation and other logistics and our government stands ready to provide all the support required," Harper said in a statement.
By mid-afternoon, rivers had flooded roads and bridges, forcing many residents of in High River and other small towns to flee by boat or helicopter.
Calgary officials said the downtown site of the Calgary Stampede rodeo was flooded, and water in the nearby Saddledome hockey arena was reportedly up to the fourteenth row of seats.
More than 100 millimetres (3.9 inches) of rain has fallen in some parts of southern Alberta in just two days, and forecasters say the precipitation won't let up until Saturday.
"The flooding situation is very acute in the foothills and the mountains," said Chris Scott, director of meteorology at The Weather Network, noting that 220 millimetres (8.7 inches), equal to nearly six months of normal rainfall, had fallen in 36 hours near Canmore in the Canadian Rockies.
"Now all that water is rushing downstream and that's why the situation is so bad in Calgary. This is an unprecedented flooding event."
Alberta lies to the east of the Rockies, and many parts of the province are normally very dry.
But a stubborn area of high pressure in Alaska and northern Alberta has pulled a stream of moisture up into southern Alberta from the United States, causing the heavy downpours.
Alberta's oilfields lie to the north of the flooded areas, and energy output has not been affected. Provincial Municipal Affairs Minister Doug Griffiths said there was no threat to Alberta's network of oil and natural gas pipelines.
Some agricultural areas were also flooded, and while crops will likely recover, they will be more vulnerable.
"Any organism that has got stress has lower immunity, so they're going to be vulnerable to fungal diseases and insect pests," said Neil Whatley, a crop specialist at the Alberta government.
Alberta produces the second-largest volumes of wheat and canola among Canadian provinces.
Television images showed trees and debris being swept down the swollen Bow River, which flows through central Calgary and which crested at around 1,500 cubic metres (396,000 U.S. gallons) per second overnight, more than five times the normal flow rate for this time of year.
The TransCanada Highway, the country's main east-west artery, was closed at Canmore after Cougar Creek burst its banks.
Flows on the Bow, and on the Elbow River which flows into it, were around three times as high as during the last serious Calgary floods in 2005, which caused an estimated C$400 million ($384 million) worth of damage.
No figures were available for damage from the latest floods.
In Calgary, Mayor Naheed Nenshi urged people to stay at home on Friday. Schools were closed, as well as many offices in the downtown core, where lights were off and few people on the streets.
Suncor Energy Inc and Imperial Oil Ltd said staff in their Calgary headquarters had been told to stay at home. Shorcan Energy Brokers, which provides live prices for many Canadian crude grades, was trading out of Toronto on Friday rather than at its usual Calgary base.
Road closures on many of the city's main roads caused traffic gridlock, prompting some to abandon their vehicles.
Despite warnings from the authorities to stay away from rivers, people congregated on those bridges that remained open over the Bow River to take pictures of the rising flood waters. Trees and other debris floated in the churning river, which had in places burst its banks.
Calgary Zoo has a contingency plan to move its lions and tigers into holding cells in a Calgary jail if the flood waters threaten the animals' quarters but there is now little risk it will be forced to take the step.
While the much of the zoo, which lies on an island in the Bow River, has been flooded, the zoo said it has been able to accommodate most of its threatened animals on higher ground within its property, while two pot-bellied pigs were moved to the city's animal shelter and two zebras were taken to a wildlife conservation center south of Calgary.