Lighter snowpack may reduce flood risk this spring
Spring flood season is still two months away, but for the moment the region has a little good news: there’s a lot less snow upriver from Ottawa and Gatineau than there was last winter.
A heavy snowpack wasn’t the single biggest factor in last May’s floods, says a report from the Ottawa River Regulation Planning Board. But it played a role.
“The main cause of the exceptional 2017 spring flooding can be described easily in just a few words: rain, rain and even more rain,” the report says.
This rain coincided “with melting snow that had already saturated the ground and swollen waterways, generating exceptional volumes of water in the Ottawa River basin.”
Of all the risk factors, the only one that can be seen well in advance is the amount of snow waiting to melt. In 2017 the snow cover across the upper part of the Ottawa watershed was deeper than average.
However this year the snow depth is about average in the northern areas and “well below average” in the southern part of the watershed, says the Ottawa River Regulation Planning Board.
This is based on figures from the beginning of February, before the recent melting.
In the Mississippi River watershed north and west of Arnprior, for instance, the “water equivalent” — the amount of water you would get if you took all that snow and melted it — ranges from one to 25 millimetres in the area up to Pembroke, and from 25 to 50 mm in the upper parts of the watershed, significantly below normal.
But Jennifer North of the Mississippi Valley Conservation Authority cautions that the impact of our recent thaw is less than it might appear.
“With this melt, we’re not seeing a large amount of run-off,” she said. “The majority of its is ponding, so it’s staying on the ground ... It’s kind of in limbo.”
Still, there’s less water waiting to come downstream from the huge watershed that stretches up past Temiskaming and on into Quebec.
The report on the spring flooding of 2017 traces out a perfect storm of factors that combined to push the river to record heights.
“On average, the total precipitation in April and May is close to 150 mm,” it says. “In 2017, the precipitation totalled an exceptional 257 mm over the entire basin in April and May.”
The Ottawa River peaked at Britannia May 6, at Gatineau May 7 and at the Carillon dam May 8, beating previous flood years of 1974 and 1976. At Britannia it was the highest water in 102 years, two full metres higher than the river level measured this week (58.41 m).