(Adds details on Calgary evacuation, possible deaths)
By Nia Williams and Scott Haggett
CALGARY, Alberta, June 21 (Reuters) - The heaviest floods in decades shut down the Canadian oil capital of Calgary on Friday, with an evacuation of the downtown core and tens of thousands of residents forced to leave their homes.
Police said four people may have died in the town of High River, located about 60 km (37 miles) south of Calgary, including a woman who was swept away with her camper and a man seen falling from a canoe in the swollen Highwood River.
Some 1,200 troops were helping with the evacuations.
"This is a tragic situation," Alberta Premier Alison Redford said at a press conference. "I think in the circumstances the response has been incredibly effective but that doesn't mean that people aren't impacted. This is a natural disaster."
Around 100,000 people of Calgary's 1.1 million residents, were ordered to leave their homes, while smaller communities were evacuated elsewhere in the Western Canadian province.
By mid-afternoon, rivers had flooded roads and bridges, forcing many residents 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 rain 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, nearly half a year's worth of rain, 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 rainfall.
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.
Pictures 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 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 Nenshi urged people to stay at home. 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 floodwaters. 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. Nenshi said that had not been needed yet.
($1=$1.04 Canadian) (Additional reporting by Randall Palmer and Rod Nickel, additional writing by Cameron French; Editing by Janet Guttsman, Bernadette Baum and Peter Galloway)