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Our Gem: Managing water levels in Coeur d’Alene Lake

| February 28, 2021 1:00 AM

Coeur d’Alene Lake was created after the last period of glaciation by a natural restriction at its outlet, the start of the Spokane River. In the 1890s, Fredrick Post constructed a mill and three dams nine miles downstream on the River, at what is now called Post Falls. Avista purchased the site and rebuilt the dams, providing hydropower beginning in 1906. The three dams span between two islands, connecting the north and south shores of the river. The north and south channel dams are spillways and the middle channel dam includes the powerhouse, where water is passed through six turbines to spin generators to make electricity. For simplicity, people refer to the Post Falls “Dam,” rather than three dams. Many people are surprised to learn that the Post Falls Dam only affects the lake’s elevation and Spokane River flows five to seven months out of the year. Every year is different due to nature’s variability.

Each year during winter and spring, the lake level and downstream river flows can fluctuate widely, all due to a combination of temperature, precipitation and snowmelt. During this time, whatever water flows out of the lake simply passes through the Post Falls Dam, and the lake reaches its own natural elevation.

This free flow continues until the end of spring runoff, which is typically between late May and early July, depending on snowpack and weather. Once spring runoff flows recede and diminished flows into the lake are forecast, Avista can close spill gates at the dam. This begins to fill the nine-mile stretch of the Spokane River between Coeur d’Alene Lake and Post Falls Dam.

Avista regulates flows until the lake elevation is at or near 2,128 feet, and forecasts indicate that the remaining snowpack or forecasted storms will not push the lake above this level. Throughout the summer, Avista adjusts the river flow rate to match inflows, keeping the lake at a steady elevation.

During the summer, Avista is required to release at least 600 cubic feet per second (cfs) of river flows from the Post Falls Dam at all times. If the elevation of Coeur d’Alene Lake drops more than three inches below summer level, the flows must be reduced to 500 cfs. Following the Tuesday after Labor Day, Avista begins to draw water from the lake, both to generate electricity and to provide room in the lake for rain or other precipitation. Avista cannot cause the lake to go above 2128 feet by its dam operations. When the lake and river are on free-flow conditions, however, the lake sometimes rises as much as seven feet or more above 2128 feet, as inflows greatly exceed the natural outflow capacity of the Spokane River.

You can access water flow information, and notification of anticipated elevation changes on Lake Spokane, the Spokane River and Coeur d’Alene Lake at myavista.com/waterflow or on a 24-hour telephone information line: (208) 769-1357 in Idaho or (509) 495-8043 in Washington. The recorded information is provided to advise shoreline property owners and commercial and recreational users of changes in the lake and river elevation levels that may affect plans for water use. Keep an eye out for high-water events to avoid losing belongings, yard waste, or other pollutants to the water.

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This Our Gem article is authored by Meghan Lunney, Avista Spokane River License Manager.

The Our Gem Coeur d’Alene Lake Collaborative is a team of committed and passionate professionals working to preserve lake health and protect water quality by promoting community awareness of local water resources through education, outreach and stewardship. Our Gem includes local experts from the University of Idaho Community Water Resource Center, Coeur d’Alene Tribe Lake Management Department, Idaho Department of Environmental Quality, Kootenai Environmental Alliance, Kootenai County, Coeur d’Alene Regional Chamber and CDA 2030.