PPL to drop lake levels

By ALEX STRICKLAND

Bigfork Eagle

The water level in Flathead Lake will be dropped to 2,891 feet by Nov. 1 to reduce shoreline erosion, according to a PPL Montana spokesman.

Lake levels have historically averaged around 2,892.2 feet on Nov. 1, PPL Montana director of hydro licensing and compliance Jon Jourdonnais said.

That one foot drop makes an enormous difference in shoreline erosion as winter sets in and the lake is buffeted by some of the most severe weather of the year.

University of Montana scientist Mark Lorang said the draw down is a reaction to a study he conducted on the lake over a 20 year period in which he observed severe shoreline erosion — especially on the north shore — when lake levels stayed high into the late fall and winter.

"At 2,891, it drops the level of water to two or three inches at the shoreline so it makes waves break hundreds of yards offshore," he said.

Lorang said the laws of physics dictate how much wave energy can be taken into shallow water, as rule, a wave's height cannot exceed about 80 percent of the water's depth.

That means that a few inches of water only allow a tiny wave to reach shore and that large waves rolling in from deep water are forced to break as the lake bed rises up.

Lorang said that dropping the water level also helps prevent damage to docks and other property that is the result of high waves and — later in the season — ice.

Before Kerr Dam was built, Lorang said the lake levels would have naturally dropped toward the end of summer as the effects of spring runoff dissipated. By mid-August the lake would be at about 2,888 feet, he said. That three feet of vertical difference may not seem like much, but it can mean hundreds of yards of beach suddenly dry. In fact, Lorang said some long-time residents have told him about days when there was an uninterrupted sandy shoreline from Bigfork to Somers.

"My research was comparing the natural regime to the regulated regime and how we could make that a better match," he said. "The biggest impact of that would be the fall draw down."

Another benefit of a gradual draw down started earlier in the season — like this year's — is the positive effect it has downstream, Lorang said.

In the past there would be a more rapid release of water from Flathead Lake in November and December which pumped up the river level and adversely affected aquatic life that had been in naturally low water levels through the end of summer.

Lorang, who studied wave dynamics on the Oregon coast early in his career, compared effects lowering Flathead Lake by three feet to lowering the depth of the Pacific Ocean at the Oregon Coast by 25 feet.

The bottom line, he said, is that the fall draw downs can reduce 80 to 90 percent of total wave energy from ever reaching the shore and that makes a big difference.

"It's a big positive thing."

PPL Montana owns and operates Kerr Dam, about five miles south of Polson.