Last week's rapid runoff filled Lake Koocanusa to a point where the U.S. Army Corps of Engineers intends to run Libby Dam at full powerhouse capacity for the next few weeks.

The Corps issued a press release announcing that all five turbines will operate at maximum capacity for a release of 25,000 cubic feet per second.

Those flows are expected to continue "for several weeks in order to slow the refill and allow storage volume in Lake Koocanusa to accommodate as much of the high elevation runoff as possible to minimize the risk of Kootenai Valley flooding and Libby Dam spilling," the release states.

Hot weather, including some record highs at Libby and Eureka, caused inflows at Lake Koocanusa to rise rapidly last week, peaking at nearly 80,000 cfs during the weekend.

"We had some pretty good inflows," said Mick Shea, the Libby Dam project superintendent. "You have to start moving water in order to make room for storage."

Maximum powerhouse releases are unusual for Libby Dam, but only in the context of recent low-water years. Shea said this is the first "average" water year since 2002 in the Kootenai basin.

Total inflows at Lake Koocanusa are expected to approach the historic average of 6.23 million acre feet. Inflows were 4.7 million acre feet in 2003, 4.7 million in 2004 and 5.57 million last year.

"We haven't seen normal water years in a few years, so that kind of skews our memories," Shea said. "In low water years, you're often conserving water."

Lake Koocanusa has risen to an elevation of 2,445 feet, not far from the full pool elevation of 2,459.

"We saw the storage diminish during that hot weather," Shea said. "We've basically got 13 feet of storage in the pool left. But we're also seeing inflows taper off, and we've got 60 degree weather ahead of us, which is going to suppress inflow."

Although a healthy snowpack produced a typical water year, the Corps said the rapid runoff surely would have caused major flooding in the Kootenai Valley if it weren't for Libby Dam.

Kootenai River flows would have reached 100,000 cfs, "and the towns of Libby, Troy and Bonners Ferry would have been flooded," according to the press release. "Without Libby Dam flood control, Bonners Ferry and the Kootenai Flats area would have seen flooding similar to that experienced in 1954 and 1961."

Libby Dam became operational in 1975.

Reporter Jim Mann may be reached at 758-4407 or by e-mail at jmann@dailyinterlake.com.