Southern Alberta is no stranger to flooding, but this week's devastation from Canmore to Calgary and beyond was the result of a unique confluence of unexpected weather and a still partially frozen landscape unable to soak up the unprecedented deluge.

Hydrologists who watch the waterways like the Bow and Elbow rivers say several factors were at play since the rain started to fall about four days ago.

"To have these very large flood events … the stars have to line up," says Uldis Silins, a hydrologist at the University of Alberta in Edmonton.

Most significant is a large amount of rainfall — up to 200 millimetres in some places. Add in ground that is already saturated because of some more modest precipitation — about 40 millimetres — preceding the deluge. Combine that with areas that were still frozen not far below the surface and a local geography that encourages water to run down hill quickly, and there's a recipe for this week's devastation.

"In this particular case, it was a little bit of precipitation preceding the heaviest rainfall, and then a period of 16 hours of very heavy rainfall in the Elbow," says Silins.

"In the Crow's Nest Pass, it was about 12 hours of very heavy rainfall on top of already wet ground.

"And then in the case of the Elbow, that's occurring in the headwaters of a very steep watershed, and so those flows then are routed down fairly quickly down valley. In the case of Calgary, they arrived fairly quickly."

The deluges were the result of some unusual weather. Along with the torrents of rain, there were unexpected wind patterns and the convergence of two huge weather systems.

Some of the hardest-hit areas have experienced twice as much rain in 48 hours than the normal average for all of June.

On June 20, Calgary experienced record one-day rainfall with 45 millimetres coming down. The previous record was 35.1 millimetres, set in 1964.

Odd circumstances

The heavy rain is also the product of an odd set of circumstances, says Stephanie Barsby, CBC's meteorologist in Edmonton.

The massive weather system responsible for the storms was still trapped over southern Alberta on Friday by a high-pressure system to the north and winds blowing toward the west, the opposite direction of the prevailing winds throughout Canada.

"That high pressure system is preventing the storm from moving north, and the Rockies are preventing it from moving west, so it's stuck right over the regions that are seeing the flooding," said Barsby.

"It's unusual to see a system stuck in one place for such an extended period of time."

Timing was also a factor in this week's events.

"Things evolved quickly because of the time of year," says John Pomeroy, a University of Saskatchewan hydrologist who was trapped in his home in Canmore on Friday after the conditions forced the closure of the Trans-Canada Highway and isolated the mountain resort town.

Pomeroy was safe — as a hydrologist, he prefers a house on a hill well above the river — but he's been watching with intense interest the confluence of events that culminated in this week’s floods.

"There's still some snow in the mountains so the ground was frozen and things are relatively wet this time of year already," he said. And then the heavy rain moved in.

For Pomeroy, who had his local research basin — Marmot Creek in the Kananaskis Country park system — destroyed by high flows on Thursday, it all came as a surprise.

"The interesting thing is that a week ago, you wouldn't have seen this or forecast this as a particularly large event." he says.

"The snowpacks were sort of average. The snowmelt had been a little bit late in the mountains, but nothing spectacular."

Then came the deluge.

"We know it was raining on the tops of the mountains and that causes rapid snow melt, and so we think in some places another 110 millimetres of snowmelt water contributed to that rainfall water," says Pomeroy.

Soil that started off reasonably dry within half a day was saturated and no longer able to soak up any moisture.

"The other thing is that one of my colleagues observed that some of the soils were still frozen 50 centimetres under the ground so they couldn't absorb much water," says Pomeroy.

'Eroding new channels'

All that means the water has to start going somewhere else.

"It starts moving overland and eroding new channels, which can be very destructive and so that's what’s happening," says Pomeroy, who said he was at a spot on Cougar Creek on Monday that you could jump over. By Friday, the water was 100 metres wide at that same place.

Pomeroy says the precipitation and stream flow measured in some areas this week are the highest ever recorded.

"It's certainly beyond what any town or municipality or province would reasonably deem necessary for flood protection.

"We have flood-risk maps for Canmore and they show areas that aren't supposed to get flooded, and they're all flooded right now."

For Pomeroy, this week's experience has been a unique juxtaposition of the theoretical aspects of his professional life and the very real toll the forces of nature can take on people.

On any given day, he might spend 12 or 14 hours a day thinking about how water flows. He might work on computer models or statistics for high-flow events.

"They can become numbers and scientific methods in your mind and it's really different then to start to see it happening or start to see houses going down and massive destruction in your own community, and talk to people who have had terrible things happen.

"It puts a human face on those numbers, so in that sense it inspires us to do an even better job and work harder at this and try to come up with better solutions for everyone."

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  • Homeowner Eddy Marshall becomes emotional as he surveys the damage to his basement at his residence in High River, Alberta on Monday, July 1, 2013. A second wave of homeowners and business owners were allowed to return to the flood ravaged town since the disaster struck on June 20. THE CANADIAN PRESS/Jordan Verlage

  • Mike Sojer moves clothing and items from his family's clothing store as clean up work commences on downtown places of business in High River, Alberta on July 1, 2013. A second wave of homeowners and business owners were allowed to return to the flood ravaged town since the disaster struck on June 20. THE CANADIAN PRESS/Jordan Verlage

  • A resident walks past a motor boat lying in the street in High River, Alberta on Monday, July 1, 2013. A second wave of homeowners and business owners were allowed to return to the flood ravaged town since the disaster struck on June 20. THE CANADIAN PRESS/Jordan Verlage

  • Homeowner Eddy Marshall carries belonging from his basement at his residence in High River, Alberta on Monday, July 1, 2013. A second wave of homeowners and business owners were allowed to return to the flood ravaged town since the disaster struck on June 20.THE CANADIAN PRESS/Jordan Verlage

  • Mud and building contents litter the sidewalk as clean up work commences on downtown places of business in High River, Alberta on Monday, July 1, 2013. A second wave of homeowners and business owners were allowed to return to the flood ravaged town since the disaster struck on June 20. THE CANADIAN PRESS/Jordan Verlage

  • A pile of debris sits outside a Town of High River Municipal building as clean up work commences on downtown places of business in High River, Alberta on Monday, July 1, 2013. A second wave of homeowners and business owners were allowed to return to the flood ravaged town since the disaster struck on June 20. THE CANADIAN PRESS/Jordan Verlage

  • A volunteer's boots are caked with mud as clean up work continues on downtown places of business in High River, Alberta on Monday, July 1, 2013. A second wave of homeowners and business owners were allowed to return to the flood ravaged town since the disaster struck on June 20. THE CANADIAN PRESS/Jordan Verlage

  • Water surrounds homes in a flooded neighborhood in High River, Alberta, Canada on Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods over a week earlier. (AP Photo/The Canadian Press, Jeff McIntosh)

  • Water floods a neighborhood in High River, Alberta, Canada on Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods over a week earlier. (AP Photo/The Canadian Press, Jeff McIntosh)

  • A residents cleans up his home in High River, Alberta, Canada on Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods over a week earlier. (AP Photo/The Canadian Press, Jeff McIntosh)

  • Water floods a neighborhood in High River, Alberta, Canada on Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods over a week earlier. (AP Photo/The Canadian Press, Jeff McIntosh)

  • Waters flood a neighborhood in High River, Alberta, Canada on Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods more than a week ago. (AP Photo/The Canadian Press, Jeff McIntosh)

  • Heaps of damaged goods pile up on High River streets, as residents are finally allowed to return.

  • Resident Christine Doefel wipes away tears as she leaves a reception centre after getting entry permits and clean-up kits from the Red Cross in High River, Alta., Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods more than a week ago.THE CANADIAN PRESS/Jeff McIntosh

  • A Red Cross volunteer helps residents as they leaves a reception centre after getting an entry permit and a clean-up kit from the Red Cross in High River, Alta., Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods more than a week ago.THE CANADIAN PRESS/Jeff McIntosh

  • Residents Christine Doefel, left, and her daughter Brooklynn Carney, embrace as the leave a reception centre after getting entry permits and clean-up kits from the Red Cross in High River, Alta., Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods more than a week ago.THE CANADIAN PRESS/Jeff McIntosh

  • Residents leave a reception centre after getting entry permits and clean-up kits from the Red Cross in High River, Alta., Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods more than a week ago.THE CANADIAN PRESS/Jeff McIntosh

  • Residents embrace as they leave a reception centre after getting an entry permit and a clean-up kit from the Red Cross in High River, Alta., Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods more than a week ago.THE CANADIAN PRESS/Jeff McIntosh

  • A residents leaves a reception centre after getting an entry permit and a clean-up kit from the Red Cross in High River, Alta., Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods more than a week ago.THE CANADIAN PRESS/Jeff McIntosh

  • Residents leave a reception centre after get entry permits and clean-up kits from the Red Cross in High River, Alta., Saturday, June 29, 2013. The Alberta government is letting people who live in High River return to their homes in stages after being forced out by floods more than a week ago.THE CANADIAN PRESS/Jeff McIntosh

  • Residents wait in line to get entry permits to the flooded town of High River, Alta., Saturday, June 29, 2013. The Alberta government is letting people who live in High River to return to their homes in stages after being forced out by floods more than a week ago. THE CANADIAN PRESS/Jeff McIntosh

  • Al Moore, a 42-year-resident of High River, listens to the radio broadcast on Friday, June 28, 2013 of the new re-entry plan for evacuees northwest of High River, Alberta. THE CANADIAN PRESS/Jordan Verlage

  • The Calgary flood as seen from space.

  • The Calgary flood as seen from space compared with a before-and-after photo.

  • Calgary mayor Naheed Nenshi, middle, and Calgary Stampeders president Gord Norrie, 2nd right, and others during pregame cerimonies honoring Alberta Flood Relief first responders and find raising efforts prior to CFL action between Calgary and BC in Calgary, Alberta on Friday, June 28, 2013. (CFL PHOTO - Larry MacDougal)

  • Calgary Stampeders Jon Cornish leaps with the ball during second half CFL action against the BC Lions in Calgary, Alberta on Friday, June 28, 2013. Cornish pledged $2,000 to Alberta flood relief immediately following the game. (CFL PHOTO - Larry MacDougal)

  • Cleanup continues on the Calgary Stampede grounds one week after major flooding in Calgary, Alberta on Thursday, June 27, 2013. The Elbow River flows by on to top and left, Saddledome is situated lower right and the chuckwagon track and rodeo grounds are top.

  • This photo provided by Transportation Safety Board shows a train with derailed tankers at the Bonny Brook bridge in Calgary, Alberta Canada on Thursday, June 27, 2013. The City of Calgary says conditions have stabilized at the site of the bridge collapse that caused six cars to derail and start sinking into the Bow River. Canadian Pacific Railway says five of the cars are carrying a petroleum product used to dilute raw oilsands bitumen. The city's acting fire chief has said crews are stringing a cable through the railcars and securing it to bulldozers on land. The industrial area near the bridge has been evacuated and booms are being deployed down river in case of any spills. There are no homes nearby, but several business are effected. (AP Photo/Transportation Safety Board via The Canadian Press)

  • Monica West carries damaged goods out of a souvenir shop as clean-up crews work at the Calgary Zoo in Calgary, Alberta, Canada on Tuesday, June 25, 2013. Heavy rains caused flooding, closed roads, and forced evacuations across Southern Alberta. Calgary Zoo staff risked their lives over the weekend to stop a hippo from escaping into the swollen Bow River. Director of animal care, conservation and research, Jake Veasey, says flood waters in the hippos' enclosure rose high enough during the floods for the dangerous herbivores to swim out. One hippo named Lobi was feeling particularly adventurous and was moving freely around the African Savannah building. Veasey had to break a window to get into the building and he swam in the muddy flood waters to find the animal. Zoo staff used cinder blocks and construction equipment to block Lobi inside the building. (AP Photo/The Canadian Press, Jeff McIntosh)

  • A muddied research book dries as clean-up crews work at the Calgary Zoo in Calgary, Alberta, Canada on Tuesday, June 25, 2013. Heavy rains caused flooding, closed roads, and forced evacuations across Southern Alberta. Calgary Zoo staff risked their lives over the weekend to stop a hippo from escaping into the swollen Bow River. Director of animal care, conservation and research, Jake Veasey, says flood waters in the hippos' enclosure rose high enough during the floods for the dangerous herbivores to swim out. One hippo named Lobi was feeling particularly adventurous and was moving freely around the African Savannah building. Veasey had to break a window to get into the building and he swam in the muddy flood waters to find the animal. Zoo staff used cinder blocks and construction equipment to block Lobi inside the building. (AP Photo/The Canadian Press, Jeff McIntosh)

  • A soldier walks by a pile of speedboats in High River, Alberta on Tuesday, June 25, 2013. High River was hit by a devastating flood on June 20 which caused a mass evacuation of the entire town although some residents chose to defy the order. Alberta's premier pledged $1 billion on Monday to help people recover from the floods. (AP Photo/The Canadian Press, Jordan Verlage)

  • A road crew foreman surveys the washed-out lanes of northbound MacLeod Trail in Calgary, Alta., Monday, June 24, 2013. Heavy rains caused flooding, closed roads, and forced evacuations across Southern Alberta. (AP Photo/The Canadian Press, Jeff McIntosh)

  • Clean-up crews work at the Calgary Zoo in Calgary, Alberta, Canada on Tuesday, June 25, 2013. Heavy rains caused flooding, closed roads, and forced evacuations across Southern Alberta. Calgary Zoo staff risked their lives over the weekend to stop a hippo from escaping into the swollen Bow River. Director of animal care, conservation and research, Jake Veasey, says flood waters in the hippos' enclosure rose high enough during the floods for the dangerous herbivores to swim out. One hippo named Lobi was feeling particularly adventurous and was moving freely around the African Savannah building. Veasey had to break a window to get into the building and he swam in the muddy flood waters to find the animal. Zoo staff used cinder blocks and construction equipment to block Lobi inside the building. (AP Photo/The Canadian Press, Jeff McIntosh)

  • Wreckage lies along Center Street in High River, Alberta on Tuesday, June 25, 2013. High River was hit by a devastating flood on June 20 which caused a mass evacuation of the entire town although some residents chose to defy the order. Alberta's premier pledged $1 billion on Monday to help people recover from the floods. (AP Photo/The Canadian Press, Jordan Verlage)

  • Calgary firefighters check on homes as residents and volunteers are in clean up mode in the community of Bowness as most of the homes have been pumped out with still some remaining submerged in water in Calgary, Alberta, on Monday, June 24, 2013. Alberta's premier pledged $1 billion on Monday to help people recover from floods that devastated parts of the western Canadian province. (AP Photo/The Canadian Press, Nathan Denette)

  • A road crew foreman surveys the washed-out lanes of northbound MacLeod Trail in Calgary, Alta., Monday, June 24, 2013. Heavy rains caused flooding, closed roads, and forced evacuations across Southern Alberta. (AP Photo/The Canadian Press, Jeff McIntosh)

  • Thaya Gallant

    Thaya Gallant helps with the flood clean-up at a law office in Calgary, Alta., Monday, June 24, 2013. Heavy rains caused flooding, closed roads, and forced evacuations across Southern Alberta. (AP Photo/The Canadian Press, Jeff McIntosh)

  • Calgary firefighters check on home as residents and volunteers are in clean up mode in the community of Bowness as most of the homes have been pumped out with still some remaining submerged in water in Calgary, Alberta on Monday, June 24, 2013. Alberta's premier pledged $1 billion on Monday to help people recover from floods that devastated parts of the western Canadian province. (AP Photo/The Canadian Press, Nathan Denette)

  • People watch as the river peaks, causing flooding in Medicine hat, Alta., on Monday, June 24, 2013. Officials in Medicine Hat said Monday they believe water levels on the South Saskatchewan River have peaked and that flooding won't be as severe as initially feared. Roughly 10,000 people were evacuated as the city of 60,000 prepared for the surge of water that swamped Calgary and surrounding areas last week. (AP Photo/The Canadian Press, Nathan Denette)

  • Residents and volunteers are in clean up mode in the community of Bowness as most of the homes have been pumped out with still some remaining submerged in water in Calgary, Alberta on Monday, June 24, 2013. Alberta's premier pledged $1 billion on Monday to help people recover from floods that devastated parts of the western Canadian province. (AP Photo/The Canadian Press, Nathan Denette)

  • Lisa Nguyen

    Resident Lisa Nguyen, right, cleans of the mud from photographs and negatives as volunteer Jacinta Babbitt, left, shows a clean picture of Nguyen when she was younger during flood clean up in the community of Bowness in Calgary, Alberta on Monday, June 24, 2013. Alberta's premier pledged $1 billion on Monday to help people recover from floods that devastated parts of the western Canadian province. (AP Photo/The Canadian Press, Nathan Denette)

  • Residents and volunteers are in flood clean up mode in the community of Bowness in Calgary, Alberta on Monday, June 24, 2013. Alberta's premier pledged $1 billion on Monday to help people recover from floods that devastated parts of the western Canadian province. (AP Photo/The Canadian Press, Nathan Denette)

  • Daniel Boddy, nine, helps with the clean-up work as hose lines from pumps drain water from flooded basement in the Bridgeland neighborhood in Calgary, Alta., Sunday, June 23, 2013. Heavy rains caused flooding, closed roads, and forced evacuation across Southern Alberta.

  • Daniel Boddy, nine, helps with the clean-up work as hose lines from pumps drain water from flooded basement in the Bridgeland neighborhood in Calgary, Alta., Sunday, June 23, 2013. Heavy rains caused flooding, closed roads, and forced evacuation across Southern Alberta.

  • A boy crosses a silt covered street in Chinatown as clean-up crews work in Calgary, Alta., Sunday, June 23, 2013. Heavy rains caused flooding, closed roads, and forced evacuation across Southern Alberta.

  • People place sand bags as the river rises rapidly and begins flooding in Medicine Hat, Alberta, on Sunday, June 23, 2013.

  • Homeowner Glenn Tibbles looks at the damage done by floodwaters to his home near downtown Calgary, Alberta, on Sunday, June 23, 2013. About 65,000 residents of Calgary were being allowed to return to their homes Sunday to assess the damage from flooding that has left Alberta's largest city awash in debris and dirty water.

  • Cpl. Brett Martens from CFB Edmonton helps a resident clear out damaged debris from their home near downtown Calgary, Alberta, on Sunday, June 23, 2013. About 65,000 residents of Calgary were being allowed to return to their homes Sunday to assess the damage from flooding that has left Alberta's largest city awash in debris and dirty water.

  • A home is inundated with floodwaters on the Siksika First Nation, Alberta, on Sunday, June 23, 2013. Alberta's municipal affairs minister says 27 communities are under a state of emergency as some areas begin to recover from flooding while others are still bracing for it.

  • People watch as the river rises rapidly and begins flooding in Medicine Hat, Alta., on Sunday, June 23, 2013. THE CANADIAN PRESS/Nathan Denette

  • Floodwaters inundate homes in Medicine Hat, Alberta, on Sunday, June 23, 2013. Alberta's municipal affairs minister says 27 communities are under a state of emergency as some areas begin to recover from flooding while others are still bracing for it.