There was so much water in the Similkameen River during a flood event last month it led to an “unprecedented” reversal of flow into Osoyoos Lake, according to a scientist attached to the international body that regulates lake levels.
After flowing through Osoyoos Lake and crossing the Canada-U.S. border, the Okanagan River normally moves through Zosel Dam and merges with the Similkameen River near Oroville, Wash., which is just a few kilometres south of the international divide.
Once the two rivers merge, water normally continues flowing south in what is known on the U.S. side as the Okanogan River, which eventually joins with the Columbia River.
But late on Nov. 15, the Similkameen River was so swollen that its water started flowing north up the Okanogan River and into Osoyoos Lake. That floodwater carried with it silt that was deposited in Osoyoos Lake, providing visual evidence of the flow reversal.
“Historically, these (flow reversals) have only previously occurred during the spring freshet period, and have never been measured in the autumn,” said Martin Suchy, Canadian section secretary for the International Osoyoos Lake Board of Control, in an email.
Andrew Gendaszek, who serves as one of Suchy’s counterparts on the U.S. side, also noted in an email that it’s not uncommon during spring freshet for the Similkameen River to act as a “hydraulic dam” and slow the flow of the Okanogan River out of Osoyoos Lake, “but it is rare for flow to reverse.”
“The occurrence of this flow reversal during the autumn is unprecedented within the historical record,” continued Gendaszek, a senior hydrologist with the U.S. Geological Survey.
And it took an unprecedented amount of precipitation to make it happen.
At 3 p.m. on Nov. 16, a USGS gauge at Nighthawk recorded the Similkameen River’s discharge at 762 cubic metres per second, nearly double the all-time high for November of 420 and not far off the record high of 883 measured in May 2018.
At the time, a flood warning was in effect for the Similkameen River as a result of an atmospheric river that deluged southern B.C.
Osoyoos Lake levels are controlled by the Zosel Dam and managed by the International Joint Commission, which was created by a Canada-U.S. treaty in 1909, and is meant to head off disputes between the two countries regarding cross-border water bodies, including the Great Lakes.