Rainfall raising Okanagan Lake’s level

Thunderstorms and runoff have raised the level of Okanagan Lake well above normal for this time of year.

The Okanagan, particularly around Mission Creek, is under a high streamflow advisory, as of mid-Wednesday, following a series of rainstorms that swept through the valley Tuesday and Wednesday.

Downpours of rain drummed on hard surfaces and left pools of water on roads, as well as swelling waterways like Mission Creek, which peaked at flows of 58 cubic metres/second early Wednesday—a one in three year event, reported Des Anderson, section head, public safety and protection in the natural resource operations ministry.

The severe thunderstorm warning issued by Environment Canada ended Wednesday, after thunderstorms and overnight showers eased up.

Mission Creek started to rise late Tuesday evening, and went from flows of 23 cubic metres per second to 58 on Wednesday.

With additional rain forecast, and runoff from the very large Mission Creek watershed still working its way into the creek and down to the valley bottom, he anticipated it would continue rising.

Okanagan Lake has also been rising and by Wednesday it was 35 centimetres above normal for this time of the year, and just 18 centimetres from full pool, a level it doesn’t generally reach until the middle of June, he reports.

Anderson said they have been releasing as much as possible from the lake system downstream since the beginning of May.

A complicating issue is that there are already issues in the south of the valley where high groundwater has flooded fields along the Okanagan River and Osoyoos Lake has risen above its target level for this time of year, in part because the Similkameen River is also very high and has backed up into Osoyoos Lake.

With the rainstorms, it’s expected to rise again.

“We’re discharging as much as possible from Okanagan Lake, balancing that with releases from Osoyoos Lake. Flows from Okanagan Lake are close to the design capacity of the channel, at 53 meters per second, in a channel designed to handle 60 metres per second of flow,” he explained.

High tributary flows in the South Okanagan are contributing even more runoff to the swollen river due to the heavy rainfall, he noted.

And, high in the Mission Creek watershed there was a further 25 millimetres of rainfall, with some snow in that, Tuesday.

“The more rain we get the less room there is in Okanagan Lake for normal snowmelt runoff,” explained Anderson.

However, the forecast for the rest of the week and weekend is for showers, which he feels the lake can handle, as long as they don’t turn into heavy rain.

The good news is that the cooler weather has slowed the rate of snow melt.

Environment Canada meteorologist Doug Lundquist said this upper low pressure system is normal for mid-May through July 7; what he calls the Okanagan’s ‘monsoon season,’ with lots of rain showers and stormy weather.

There was even snow this week on Allison Pass, the Okanagan Connector of the Coquihalla Highway and the Coquihalla itself, he said.

Actually, he said it was very dry in the Okanagan for May until the black storm clouds moved in Tuesday.

Unfortunately, he said it’s difficult to predict how much rain will come in the next few days because it simply depends on where the storms develop, but he did expect there would be another dump before Sunday or so when it’s expected this system will begin to break up and dissipate.

Deputy emergency program coordinator Brian Moore, also administration officer for the Kelowna Fire Department, said there were a couple of reports of lightning strikes Tuesday in the Black Mountain area, but fire fighters couldn’t find anything, so it’s expected the rain put them out.

Mission Creek is rising and the regional district has again closed the Mission Creek Greenway underpass at the Casorso Road Bridge.

Greenway users are advised to use caution and cross Casorso Road only when it is safe, and motorists are asked to watch for greenway users crossing the road in the area.

The trail on the south side of Mission Creek at the Gordon Drive Bridge is also closed until further notice.

People are warned that water levels may rise unexpectedly and children and pets, in particular, should be kept back from the banks of creeks which may be slippery or subject to erosion with high water.

Boaters are advised to watch for floating debris that may enter the lake as a result of the runoff.

 

jsteeves@kelownacapnews.com