Flathead Lake slowly filling up for summer

It's been a wet spring in the Flathead Valley, but with warm weather just starting to set in up in the high country, Flathead Lake is beginning to feel big effects from runoff.

"We haven't had the inflows yet," said Deb Mullowney, a spokesperson for PPL Montana, which operates Kerr Dam. "Flows are quite a bit less than typical."

As a result, the Lake hasn't filled up at its average speed. Though the large snowpack — 106 percent of average as of May 14 — and damp weather does have this spring looking familiar. It's almost identical to last year.

On May 13, 2008, the Lake was at 2,885.6 feet above sea level. This year on the same date the height was .05 feet higher.

"Last year we were a little bit later because of the potential for flooding and the late runoff," Mullowney said.

PPL Montana projects that the Lake will rise about four feet to a height of 2,890 ft. by the end of May. Mullowney said full pool — 2,893 feet — should be reached by June 15.

Though the idea of a lake with more than 100 miles of shoreline gaining four feet worth of water is mind boggling, when area rivers really get moving with snowmelt, Mullowney said inflow can far outpace outflow, even with the floodgates at Kerr Dam wide open.

Late last week, water was leaving the dam at a rate of about 15,200 cubic feet per second (cfs). For comparison, in mid-May 2008 when runoff was at its peak and area residents were worried about flooding, PPL Montana was pushing 39,000 cfs out of the Lake. But those same days, inflow from rivers and streams was close to 60,000 cfs.

"So it can really see huge flows when it gets warm," Mullowney said. "We have quite a bit to increase by the end of the month, but that's how it happens."

Each year PPL Montana raises and lowers the height of Flathead Lake, both in compliance with its operating license and in an attempt to keep the seasonal water levels near what they were before Kerr Dam was built in 1938.

The company usually starts drawing down the water level in the fall, which has the added benefit of protecting property and boat docks from damage, especially on the North Shore, which takes a beating with prevailing winds from the south.

The low water over the winter has other advantages too, besides exposing the mile-wide beach between Bigfork and Somers to intrepid walkers. It also provides an opportunity for restoration work to occur.

Mark Lorang, a scientist at the University of Montana's Yellow Bay Biological Station, has worked over the last few winters to rebuild badly eroded beaches along the Lake's northern end.