Winter ending without Flathead Lake freezing over

With spring finally in sight, it looks like Flathead Lake will make it another year without accomplishing a rare feat: Freezing over.

It is so rare, in fact, that the last time the lake froze solid was 19 years ago, during the winter of 1989-90, when a cold snap moved in from Canada and dropped the surface temperature of the lake enough that the subsequent calm resulted in lake-wide freeze ice.

That year was the third in a row during which such a cold snap visited the Flathead during the months of January and February and produced the right conditions to freeze the lake from one shore to the other.

Although a similar cold snap came through the Valley last December, dropping air temperatures to between 15 and 20 degrees below zero, Craft said those temperatures didn't last long enough to cool the lake.

"The biggest reason is just that it's a huge heat sink," Craft said. "It's the largest freshwater lake east of Mississippi, and the sheer volume of water in Flathead Lake is what helps keep the surface warm and keeps it from freezing. There is a lot of heat energy that Flathead Lake still has in December.

"It all depends on how many cold snaps you actually get in. When we get a real high wind with it, that's what gets the heat really coming off the lake fast.

"That's why you see it steam all winter long. The steam is energy leaving the lake, but there's an awful lot of it to pull out."

Craft pointed out another factor that becomes an issue with a lake as long as Flathead Lake: Wind. With 30 miles of lake from north to south, including a 25 mile stretch that is pretty much unbroken between the north shore and the Narrows, the wind is continually moving the water and feeding energy into the lake, thus keeping the surface from freezing.

"Once the lake has lost most of its heat…what you need is very cold air and high winds to super cool the water and take all the energy off the surface of the lake, to where it's actually below freezing," Craft said. "The only thing keeping it from freezing is the energy from the wind."

That's why it is necessary for a two-to-three-day period of absolute calm to follow those high winds if the lake is to freeze solid. According to Craft, the freeze must be complete in order for it to last when the wind resumes. If there are any channels where the lake remains unfrozen, the movement of the wind on the water may cause enough of a current to break up the part that did freeze.

"Once it freezes, you lose all of the energy from the wind. But you have to get it to freeze first, and that's hard to do with Flathead Lake. Swan and Whitefish lakes just don't have the mass of energy associated with them that Flathead Lake does."

Once Flathead Lake freezes from end to end, Craft said it takes until late spring or summer for it to break up. The break-up is aided by the spring runoff and the surge of water coming into the lake, which actually lifts the ice and breaks it with the help of the wind and current.

Once the ice starts breaking, it comes off fairly quickly, but the same volume of water that took so long to cool can take just as long to heat back up.

"It takes an awful lot of energy to reheat it," Craft said. "That's why people wait so long to go swimming in Flathead Lake. Sometimes it can be comfortable in late June. Sometimes, you're almost pushing August 1. This year, I don't think that there was any day that I was comfortable in it, because you had that long, drawn-out cold spell."

Even though a lake-wide freeze is a rare occurrence that hasn't happened in 19 years, it did happen three winters in a row between 1987-88 and 1989-90. Prior to that, the lake froze on average every five to seven years.

So what happened altered the normal pattern and postponed a freeze for nearly two decades?

"The one change that really stands out to me, other than general changes in weather patterns, is that they retrofitted Hungry Horse Dam to release water into the Flathead River through a selective withdrawal 'system]."

Hungry Horse Dam used to release water from the bottom of the reservoir, where the water was the coldest and most dense. The water temperature at the bottom of the reservoir hovered at about four degrees celcius most of the year, according to Craft.

With the selective withdrawal system, the water can be released from other, warmer depths, in order to match the temperature of the river. Because of the volume of water going into Flathead Lake, the lake temperature has been consistently warmer.

"They've actually gone back to more of a normal heat budget," Craft said. "That has probably put us closer to the historic cycle for lake freezes. Even Yellow Bay hasn't frozen over in a number of years, whereas it used to freeze over fairly regular."