Slow the flow

POST FALLS - For the first time, Avista Utilities has forced Lake Coeur d'Alene to fill by slowing the flow past its Post Falls Dam to raise the level of the lake.

The move reflects just how dry of a year it's been, even with moisture in April and early May.

"Normally the lake just fills on its own with the spring rain and snow melt," said Patrick Maher, Avista's senior hydroelectric operations engineer, adding that the company hasn't shut the dam completely as has been rumored.

Between April 17 and Monday, Avista raised the lake from about 5 feet below the summer level to about 3 inches below summer level of 2,128 feet, Maher said.

"From now until mid-June the lake should stay between from a quarter to half a foot below summer level," he said.

During that time, whatever flows into the lake will also flow out and down the Spokane River, Maher said. If the amount of flow down the river is greater than the amount of water that can be routed through the turbine generators in the dam (about 5,200 cubic feet per second), any excess has to be spilled over the dam.

"The more flow in the river, and hence the more spill over the dam, the lower the level of the water will be above the dam because of more open gates," he said.

The flow down the river is currently more than three times the capacity of the turbines, so twice as much water is spilling over the dam as is being used to generate electricity, Maher said. Therefore, there is more than a 5-foot water elevation difference between the lake and dam and flows are fast.

As flows decrease, the lake level will stay the same, but the water elevation at the dam will go up because fewer gates are open.

"For people using the Spokane River, they need to be careful of using the river during periods of high flow as they get farther away from the lake and closer to the dam, the depth of the water will be less," Maher said.

The drainages into the lake are low elevation and inflows tend to increase in bursts, either from rain or warm weather that quickly melts the snow.

"Residents should be aware that flows in the Spokane River will likely fluctuate between now and the middle of June," Maher said. "Lake Coeur d'Alene residents should see very little variation in the level of the lake."

Avista will keep the lake near its present level until the middle of June when it will be raised a few inches to summer level, Maher said.

"The spill gates at the dam will most likely be completely closed some time near Memorial Day," he said. "At that time, the river between the lake and the dam will be near summer levels."

April brought 133 percent of average precipitation in the Panhandle - the second-best month since October - but water volumes in area streams are still predicted to be about half of normal, according to the Idaho Natural Resources Conservation Service.

"It appears the mountains were mostly just playing catchup after having five months of dryer-than-normal conditions, Maher said.

Maher said the lack of precipitation earlier this year impacted Avista's hydro generation, but the full effect for the rest of spring and summer will depend on precipitation in the coming months.

"The dams have limited capacity to generate and, as the snow melts, if there is too much water, it is simply spilled over the top of the dams," he said.