Despite slower-than-usual runoff, officials say Flathead Lake is on pace to reach full pool by mid-June. The one thing that can push that back – and did, just last year – is the threat of flooding.
The National Weather Service, however, say flood risks look low this year. Snowpack levels are lower, and the warm-and-cool trends of the past few weeks have delivered a more relaxed release this spring compared to last year’s dramatic runoff.
Deb Mullowney, a spokeswoman for PPL Montana, said the company was on pace last week to hit 2,890 feet on May 30, and still expected being at full pool – 2,893 feet – by June 15. Those are the target dates – set for recreational purpose – in the dam’s licensing agreement for hitting those levels.
PPL Montana operates Kerr Dam under a joint license with the Confederate Salish and Kootenai Tribes. The power company and tribes, along with the U.S. Army Corps of Engineers, the Bureau of Indian Affairs and the Bonneville Power Administration, determine how quickly – or slowly – to raise the lake level each spring.
Last year, a cold, wet spring delayed the melting of snow in the mountains and pushed things back by nearly three weeks – bad news for boat owners, guide businesses and recreationists with docks in shallow bays.
“We still had a ton of snowpack last spring, so the Corp of Engineers was really concerned that if we went ahead and filled that we would get additional runoff and there would be a chance of flooding,” Mullowney said.
As a result, in 2008, the lake was still 1 1/2 feet below full pool on June 15, and didn’t entirely fill until Fourth of July weekend.
In many ways, this year’s weather has also been unusually cool, slowing runoff some. But officials say the lowered risk of flooding should keep the lake on pace for its mid-June fill.
While snowpacks across western Montana were at or slightly above average for spring, Weather Service hydrologist Ray Nickless said the high-elevation areas have seen sun. The Flathead’s snowpack has let the bulk of its moisture go, so the Middle and North forks of the Flathead River are expected to stay within their banks this spring.
“Temperatures have been similar up there this year (to last year),” Nickless said in a conference call last week. “The really big difference is that you don’t have as much snowpack.”
Mullowney said snowpack in the Flathead Basin was at 109 percent of average on on May 27, compared with 127 percent on the same date a year ago. In June 2008, snowpack was 260 percent of normal before warmer weather finally forced runoff.
As of last week, Mullowney said water was leaving the dam at a rate of about 12,700 cubic feet per second (cfs). Inflow the same day was running about 33,000 cfs. In comparison, inflow at this time last year was 51,000 cfs and outflow was around 35,000 cfs.
“That pace was short lived,” Mullowney said of 2008’s rush of water. “Still, flows are quite a bit less than typical this year.”
With more than 100 miles of shoreline, raising the lake three feet in just more than two weeks is hard to imagine. It takes about 325,851 gallons of water to cover just one acre of land to a depth of one foot.
But when the Flathead’s rivers really get moving with snowmelt, Mullowney said inflow can far outpace outflow, even with the floodgates at Kerr Dam wide open.
“You just never know from year to year what Montana weather is going to do,” she said. “Right now, we’re looking like we’re on target.”