Corps eyes small spill to avoid big spill

A controlled spill from Libby Dam is being considered starting as early as Saturday, June 10, as Lake Koocanusa nears full pool.

The U.S. Army Corps of Engineers is considering a spill of 1,500 to 1,700 cubic-feet per second to make room in the reservoir to avoid a bigger spill.

An updated forecast for runoff as of June 1, that is 10 percent higher than previously expected, has the Corps nervous since the reservoir is within four feet of full.

Mick Shea, project manager for Libby Dam, said if the Kootenai drainage receives a weather "bump" it could force the Corps to spill water to make room for the additional runoff. And the Crops' Reservoir Control Center is reporting that 50 percent of the watershed's high elevation snow pack remains in the mountains.

"If by Saturday, we are exceeding expected inflow, we will consider a small, controlled spill to make room," Shea said. "Without unseasonable heat or additional precipitation, we still think we can make it."

Modeling for runoff and releases from Libby Dam show that by June 17, Koocanusa should be within seven-tenths of a foot of full pool and inflows should decrease below the level of outflows.

"We may not have to spill," said Cindy Henriksen of Reservoir Control. "We didn't want to surprise anyone. We're monitoring the weather and seeing what the snowpack is doing."

If forecast thunderstormns don't develop over the upper basin during the last half of the week, "we won't spill," Henriksen said.

On Wednesday, the Corps was releasing 24,400 cfs from the reservoir while 33,000 cfs was coming into the reservoir via the Kootenai River in Canada. As of April 1, the drainage forecast indicated that runoff for the April through August time period was 98 percent of normal. The June 1 forecast shows the runoff to be 108 percent of normal, an increase of about a half million acre feet of water.

The limited flood control spill is expected to maintain adequate flood control storage in Koocanusa Reservoir to reduce the risk of a larger flood control spill later in June or early July. Another objective of the Corps is to avoid exceeding Montana's water quality standards for total dissolved gas saturation, when possible.

Gas bubbles are caused by water running down the spillway and hitting the river. During a controlled spill experiment that went awry in 2002, state and federal biologists learned than anything over 1,500 to 1,700 cfs caused enough gas bubbles to be in violation of the state's water quality laws, which is federal law. In the midst of the 2002 spill, the Corps was forced to increase spill to as high as 16,000 cfs after inflows from Canada increased beyond the forecast.

Much of the high elevation snow pack remains even after May's unusually warm weather and the subsequent low elevation runoff during the past two weeks, which caused near flood-stage high water in the Kootenai River below Libby Dam and flooding on the Yaak and Moyie rivers in Montana and numerous rivers in Idaho.

The Corps' water management strategy takes into account the need to provide flood protection to the region, the Corps' responsibilities concerning Endangered Species Act for listed and resident fish and aquatic life located downstream of Libby Dam, and other important local and regional considerations.