OROVILLE – Heavy rainfall and melting snowpack due to higher than normal temperatures in mid-November led to increased flows in the Similkameen River causing the Okanogan River to back up over Zosel Dam in Oroville.
On Nov. 15 discharge of the Similkameen River began to increase rapidly. This was due in part to the landfall of a Category 5 atmospheric river causing heavy, sustained rainfall and rising temperatures resulting in additional runoff from snowmelt. A flood warning for the Similkameen River was issued by the National Weather Service and the BC River Forecast Centre on Nov. 15.
Discharge peaked at the USGS Similkameen River at Nighthawk gage at 26,900 cubic feet per second (cfs) at 3 p.m. on Nov. 16. The previous historical maximum for the month of November during the 92-year record at this gage was 14,400 cfs (420 cms) on Nov. 30, 1995. The all-time historical maximum discharge of the Similkameen River at Nighthawk was 31,200 cfs on May 10, 2018 during the spring freshet.
The record high autumn discharge of the Similkameen River coupled with low Osoyoos Lake levels and minimal inflow of the Okanagan River into Osoyoos Lake, created a reversal of flow in the Okanogan River downstream of Osoyoos Lake. In the late evening on Nov. 15, the flow of the Okanogan River at Oroville reversed direction. At approximately 2 p.m. on Nov. 16, the river level downstream of Zosel Dam crested the dam’s spillway and began flowing up the channel towards Osoyoos Lake.
The upstream flow of the Similkameen River is apparent from upstream migration of the muddy Similkameen River water in comparison to clear Osoyoos Lake water in an aerial photograph by Mary Davisson, Fisheries Biologist, Confederated Tribes of the Colville Reservation. The photo shows that muddy Similkameen water reached Osoyoos Lake by 11:15 a.m. on Nov. 17, according to the International Osoyoos Lake Board of Control.
The International Osoyoos Lake Board of Control was established by the International Joint Commission (IJC) which ensures that outflows from Osoyoos Lake are regulated in accordance with the IJC’s orders of approval.