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Late snowmelt increases local flood risk
A late snowmelt this year has increased the chance of flooding on Osoyoos Lake, but it all depends on weather over the next few weeks.
“Seasonal flood risk is high” in several river basins including the Similkameen and is “moderately elevated” in others including the Okanagan-Kettle basins, the province’s River Forecast Centre says in its latest snow survey and water supply bulletin released May 8.
Cooler temperatures and unsettled weather in late April delayed the onset of snowmelt at higher elevations.
The snow basin index is 172 per cent of normal in the Similkameen basin and 128 per cent of normal in the Okanagan-Kettle basin.
Adverse weather conditions could pose a flood risk, the report said, noting also that above normal temperatures are likely in the May 15 – 22 period.
“If warmer weather occurs into the middle of May, increased snowmelt and a period of higher river levels can be expected,” the report says.
“Weather during the snowmelt season is a critical factor in determining if flooding will occur. Adverse weather, including extended periods of heat or high intensity or prolonged precipitation, can result in flooding in years with normal, or even below normal snow packs.”
The Similkameen River flows into the Okanogan River just downstream from Oroville, WA, but when water levels on the Similkameen are high, water backs up at the Zosel Dam impeding the outflow of water from Osoyoos Lake.
If water from the Okanagan River upstream is flowing at an increased rate into Osoyoos Lake, this can cause the lake’s level to rise and flood surrounding properties.
“The longer that we have snow on the ground as we get into the time of year where hotter weather is likely, that does push the risk up as well,” said Dave Campbell, head of the River Forecast Centre in an interview Friday.
On Monday, the water level on Osoyoos Lake was at 911.72 feet above sea level, up nearly a foot since May 1.
Discharge on the Similkameen River near Nighthawk, WA rose sharply with hotter temperatures the first three days in May, though it peaked at around 10,000 cubic feet per second on May 4 and has gradually declined since.
These levels are well below those recorded during flooding last year when Osoyoos Lake reached a level of 914.19 feet on May 15 and discharge on the Similkameen at Nighthawk was 20,600 cubic feet per second on May 13.
Last year’s floods were earlier in the season than normal.
“I think this year if we’re to get fairly normal weather patterns and steady melt then we’ll be fine,” said Campbell. “If we get something extreme, that higher snowpack might come into play for increased risk.”