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Canmore flood surprises province

Hydrologist calls building on flood plains irresponsible as climate change brings increased rainfall



Responding to some concerns shared by one of his post-doctoral students, Canmore resident Dr. John Pomeroy was driving around town at midnight as Wednesday rolled into Thursday, June 20.

As director of the University of Saskatchewan's hydrology department and Canada research chair in water resources and climate change, Pomeroy and his team have been maintaining streamflow and snowmelt research sites in Kananaskis Country for the past decade. Located 45 minutes' drive east of Canmore, the researchers' sites were established by Canada's federal government in the 1960s then shut down in the 1980s.

"I wanted to see what was happening at Cougar Creek, so I went to the upper and lower parts within Canmore," Pomeroy said. "I had suggested recently that it was at high risk and knew that the storm was massive and so likely to cause flooding."

Fortunately, the Town of Canmore already had diggers moving boulders in place in an effort to channel the rapidly rising Cougar Creek flow.

It might almost seem ironic that as Colorado residents struggle through intense and destructive forest fires, the storm that flooded Canmore and a massive swath of southern Alberta originated in that U.S. state.

Sucking up warm, humid air from the central US, the storm travelled north then stalled around the Montana/Alberta border. There it rotated counter-clockwise, trapped between the Rockies and a high pressure system to the north, and so its winds blew toward the west — opposite the prevailing direction. This forced the clouds to rise and cool, which then squeezed the moisture out of them — an event known as a "cold low".

"Cold lows cause most floods in the Alberta foothills," Pomeroy said.

Conditions on the ground added to the destructive force of the storm in several ways. Since rain had already fallen for a couple of days previous to the storm's arrival, the soil was saturated. In some places frozen ground was recorded just 50 centimetres down that was unable to absorb the rain. Also at higher elevations, plenty of the winter — and late spring — snowpack remained. Once the rain fell on the snow, the relatively warm humid air associated with rain condensed on the snowpack and released large amounts of energy which melted the snow very rapidly. Strong winds (felt as far as Golden, B.C., two hours' drive northwest) helped this process.

At the same time, the rainfall from the storm was unusually heavy for the Rockies. As it fell on quickly saturating and/or snow-covered slopes, gravity pushed the water overland, creating new channels with enough force to take enormous obstacles with it — from boulders to old-growth trees to backyard sheds and hot tubs, and downstream from Canmore, even houses.