Prairie watersheds depend on the mountain snow for their summer water supply, but a quick melt can be devastating
As scientists and provincial officials keep a watchful eye on the interplay between rainfall and the melting snowpack in Alberta’s Rocky Mountains this June, the shadow of the floods of 2013 looms over their calculations.
The frequency of floods such as the one that struck Calgary and area starting on June 20 eight years ago could be as low as every 60 or even 40 years, said John Pomeroy, a professor who is the director of the University of Saskatchewan’s Centre for Hydrology.
“The mountain snowpack is kind of like having extra reservoirs because it’s holding the winter precipitation back and releasing it in May, June and even July, which is great for irrigation,” he said.
“But the risk is that as long as it’s sitting up there, it’s always possible to have a large warm rainfall event and then (the combination) can cause flooding, so we have to live with both the benefits and the dangers, I guess.”
It is impossible to predict more than seven to 10 days ahead if there will be an extreme rain event, “so really, if that’s in the cards for us, I think we just don’t know it until we get closer to that,” said Clayton Weiss, Bow operations and infrastructure manager at Alberta Environment and Parks.
“We know that we will calculate the statistics and the likelihood that these things will recur, but in any given year, it’s not possible to predict those phenomena.”
However, for the reservoirs he manages, “the storage picture is very strong throughout the Bow basin … we are right on target where we need to be for this time of year.”
The Alberta side of the Rockies, also known as the Eastern Slopes, contains the headwaters for the South and North Saskatchewan river systems, which provide water for much of the prairie provinces.
“Across the Prairies, it is looking like an extreme drought right from Manitoba across (to) Alberta,” said Pomeroy during an interview May 28.
“That’s alleviated a bit now because of the recent rain, but it’s not completely alleviated, and so that’s why I’m saying demand for irrigation water in southern Alberta is probably going to stay higher than normal.”
There are typically “a few big cold low systems in the spring that dump more precipitation in the Foothills, Rockies and some of the high elevation plains,” he said.
“And so we’ve had one recently in the area, but we certainly would need a few more to keep the soil moisture conditions in the irrigated areas in good shape, and also to keep the streams going, so (we) expect a few of those to come as rainfall, but many of them even in early June as snowfalls at the highest elevations.”
Irrigation in Alberta is mostly located in the Oldman and Bow River systems, which join north of Grassy Lake, Alta., to form the South Saskatchewan River.
The snowpack in the Oldman watershed has been “highly variable, and there are some measurement locations that are well below normal,” said Pomeroy.
Automated wireless devices called snow pillows show the snowpack ranged from much below average to above average as of May 26, said Nathalie Brunet, a river forecast engineer with Alberta Environment and Parks.
Meanwhile, the Bow River basin generally ranged from average to above average, similar to the snowpack further north on the Red Deer and North Saskatchewan River basins, she said.
However, the runoff for these rivers from March to April, including the Oldman, was generally below average across the board, with the Bow River below average to much below average, she said. Cooler weather has caused the snowmelt to be delayed by a couple of weeks, said Weiss.
The positive side of the snowpack lingering in the Rockies is that it provides water for irrigation at a time when farmers need it for crops later in June, said Pomeroy. However, the longer the situation persists, so too does the danger that a large rain event could cause rain-on-snow flooding, he added.
Although the snowpack was not particularly deep in 2013, it melted slowly, meaning it was still around when it was hit by sustained rainfall, said Pomeroy. “And then the rains melted it quickly, and then the rains themselves caused flooding and added together, it was a bad combination.”
Flows in the Bow River peaked at 2,400 cubic metres per second, or about eight times its regular rate, said a City of Calgary statement. Those for the Elbow River, which joins the Bow River in downtown Calgary, were 1,240 cubic metres per second, or 12 times the typical rate.
Such flooding caused as much as $6 billion in damage across southern Alberta, resulting in five deaths and forcing the evacuation of about 80,000 people. States of emergency were declared in dozens of communities, with towns such as Canmore and High River among the hardest hit.
Several estimates have been given about the frequency of such floods on the Bow River, said Weiss. “There’s different ways of calculating it and different numbers floating around, but roughly you’d say one in 200 years.”
However, the frequency may be significantly lower, said Pomeroy, who is also director of the Global Water Futures program.
The estimate of one in 60 or even 40 years is based on research on peak flows using early records kept by authorities such as the North-West Mounted Police, now the RCMP, and the Geological Survey of Canada, he said.
The flood of 2013 would not have surprised early settlers, who endured several floods on the Bow River that were caused by heavy rain combined with the snowpack, he said. “They had a very bad sequence of floods from the 1880s right up to the 1930s.”
Things were relatively quiet until June 2005. Although conditions that spring were drier than normal, three major storms about a week apart caused flood damage across southern Alberta, with one in 10 buildings affected in Calgary, said a federal statement.
Scientists don’t know the reason why the incidence of flooding decreased on the Bow River for decades, said Pomeroy. Further complicating the picture is the fact that little is known in detail about what happened before 1885, he added.
As researchers try to estimate what the future holds based on their limited knowledge of the past, climate change is throwing a significant wildcard into the mix.
Computer simulations based on current warming trends predict it is unlikely the snowpack will be hanging around in June by the end of this century, said Pomeroy. “The rain on snow part (will be) less likely in June, but you can get heavy rains coming in April or May that might generate similar flooding.”
The way the rainfall is occurring is also different, with the incidence of multiple-day rainstorms increasing 50 percent since the 1950s across much of the Prairies, he said.
“Now, the rainfall clusters together more than it would have 50 to 80 years ago, and so we tend to get multiple-day rainstorms, whereas in the past, we’d have single-day rainstorms and some dry days and another rainstorm. And these multiple rainstorms are ones that can cause problems because the Calgary flood in 2013 was caused by four days of heavy rain.”
Moving the climate goalposts will affect the operation of irrigation systems, even if they’re not directly touched by flooding on rivers, said Pomeroy. “They rely on reliable snowpacks, and those certainly will be declining dramatically, as will the glaciers.”
He said it’s hard to imagine the snowpack will completely disappear, but winters will likely be much shorter.
“I think the concern is more the longer summer period and whether there will be enough rain to keep things green over that summer period, and then also how intense those rainfall events are, and how long drought periods are, and a lot of that is still very uncertain.”