Heavy rainfall main driver of floods – not snowpack

BOW VALLEY – A new study indicates flooding during extreme events has little to do with snowpack levels and only occurs when there is a deluge of rain.

The study by geoscience professor Cathy Ryan and her University of Calgary colleagues confirms what hydrologists expected after the 2013 southern Alberta floods that left five people dead, caused $5 billion in damages and forced 100,000 people from their homes as rivers raged.

Ryan said a historic review of hydrologic data shows the highest annual flows in rivers are by far related to rain and not significantly related to snowpack amount or the rate of melt.

“They have both been conflated because highest snowmelt and the most rainfall both occur mid- to late-June,” said Ryan, who has a home in Canmore and heard the anxiety people were expressing last week as rivers and creeks in the region swelled.

“The bottom line is watch the precipitation forecasts and don’t worry about the snowpack… flooding is caused by rain, not snow melt… when folks are fretting about the unusually high snowpack in the mountains this year, it is for naught.”

The study, titled Hydrological Behaviour of an Unregulated Eastern Slope River Under Changing Historical Climate, was published in the Canadian Water Resources Journal in April, 2022. Authors included Ryan, Yixuan Zhou, Cuauhtémoc Tonatiuh Vidrio-Sahagun, and Jianxun He.

The paper examined historical data, including flow and hydro-meteorological variables, to understand where the flow is primarily generated in the Elbow River basin, how the flow has changed, and what the main drivers are behind high flows and floods.

The study focused on the upper reaches of the 120-km long Elbow River, a tributary of the Bow River that flows out of Kananaskis Country.

Ryan said the river was ideal for examining potential climate change impacts on river hydrology because it is relatively undeveloped above Calgary’s drinking water supply at the Glenmore Reservoir.

“We chose the Elbow for this study because it is unregulated in the upper watershed – where all the river flow is generated – but the conclusions are also relevant for the Bow,” she said.

In 2013, more than 200 mm and as much as 350 mm of precipitation fell over the front ranges of the Canadian Rockies. Tributaries to the Bow River, including the Ghost, Kananaskis, Elbow, Sheep, Highwood, and many of their tributaries, all reached flood levels.

Last week over three days, Kananaskis Country saw about 135 mm of rain, although it dumped snow at higher elevations, while Banff received significantly less rainfall than the 100 mm earlier predicted.

Alberta Environment was quick to issue various flood alerts early last week, from flood warning and flood watch to high streamflow advisories over a number of days for the Bow River between Banff and Exshaw.

The Bow River and Three Sisters campgrounds were closed as a precaution by Alberta Parks and campers were relocated to other campgrounds. The water levels in the Barrier and Spray lakes reservoirs in Kananaskis Country were lowered to accommodate for all the extra rainfall.

Both municipalities of Banff and Canmore wasted no time in activating their emergency coordination centres.

The Town of Canmore set up an incident command centre at Cougar Creek, which burst its banks in 2013 and destroyed many homes.

The Municipal District of Bighorn declared a state of local emergency for the hamlet of Lac Des Arcs due to flooding concerns associated with Heart Creek. The final mitigation measures to address bank erosion along Heart Creek were completed by June 20.

As of Wednesday (June 22), the Bow River upstream and downstream of Banff has a high streamflow advisory in place. On Wednesday morning, the Bow River at Banff was running at 182.3 cubic metres per second. This compares to approximately 440 cubic metres per second during the height of the catastrophic 2013 floods.

The Pipestone River in Lake Louise remains on flood watch as of Monday afternoon. Flood watch means stream levels are rising and will approach or may exceed the banks.

“Parks Canada is actively monitoring water levels on the Bow and Pipestone rivers and staff are monitoring trails, campgrounds and other facilities and will implement closures if needed,” said James Eastham, a Parks Canada spokesperson for Lake Louise, Yoho and Kootenay field unit.

The University of Calgary’s Ryan said it is important to note that most of the river flow is generated further up into the mountains.

She said 92 to 95 per cent of the Bow River at Calgary is generated above Seebe and 94 per cent of the Elbow River is generated above Bragg Creek.

“It’s not precipitation in Calgary that drives flow in Calgary or precip in Canmore that drives flow in Canmore, it’s really precip in the upper watershed,” said Ryan.

“There’s steep, steep mountain sides without very much soil, shallow water tables, and so all the precip goes into the river reasonably quickly in the mountains, compared to the prairies.”

While June historically has the highest rainfall, Ryan said it is impossible to predict what will happen from one year to the next during the spring.

“Whether or not it will flood next year… you might as well throw a dice,” she said.

Meanwhile, the MD of Bighorn activated components of its East Exshaw high groundwater response plan on Monday afternoon following the notification from residents of water in their basements.

Pumps have been deployed in the local area and at the municipal lift station to help remove excess groundwater from the area. In addition, sandbags have been deployed for use by residents and a port-a-potty provided in the area fronting the community garden.

Road barricades and signage may be installed without notice. All residents and visitors are required to obey these closures and/or detours so that MD workers can complete necessary work tasks.

If the municipality provides separate notification of a wastewater release from the east Exshaw lift station under a boil water recommendation, residents on private wells may collect potable drinking water from the taps located at the municipal administration building during business hours.

“Residents are responsible for providing their own water containers/bottles,” said Bill Luka, the MD’s director of operations in the notice to residents.

“Water deliveries to mobility challenged residents can be arranged through our Community Services Department with a minimum four hours’ notice.”