Coeur d’Alene Lake is the second largest lake in northern Idaho, caught in a beautiful mountain setting created when great lobes of the continental ice cap receded during the last Ice Age. The lake is over 26 miles long with some 135 miles of shoreline, dotted with numerous parks, campgrounds, beaches, trails, and recreational facilities. The lake is fed primarily by two rivers, the Coeur d’Alene River and St. Joe River, and its outlet forms the Spokane River.
Reservoirs, some of which are called lakes, have a dam right at the outlet and in those instances the reservoirs are created solely by the dam. This is not the case with Coeur d’Alene Lake. Coeur d’Alene Lake is a natural lake created by a natural restriction located nine miles upstream from Post Falls Dam at the outlet of Coeur d’Alene Lake. During the winter and early spring, that restriction on the north end of the lake limits how much water can flow out of the lake.
While we control the elevation of the lake during summer and fall, you may be surprised to learn that during this time of year (spring), we do not. For about six months every year, in the winter and early spring (usually from January through June) Coeur d’Alene Lake is at natural levels. When the lake is on natural restriction, whatever water flows out of the lake passes through Post Falls Dam. During this time, we have all available spill gates open and we don’t control the level of Coeur d’Alene Lake.
There are three large tributaries (the Coeur d’Alene, St. Joe and St. Maries rivers) and many smaller streams that flow into the lake. These flows into the lake are caused solely by the weather, or a combination of temperature, precipitation, and snow melt. Almost every year, sometime between December and June, more water flows into the lake than can escape through the natural restriction at the outlet and the lake starts to rise.
The elevation of Coeur d’Alene Lake affects the elevation of the Coeur d’Alene River up to Cataldo, about 32 miles upstream of the lake, and the St. Joe River all the way up to St. Joe City also about 32 miles upstream of the lake. When there are high flows in the two rivers, there is an effect similar to what happens with Coeur d’Alene Lake. Flows increase and the water can’t escape so the rivers’ elevations increase upstream are higher than they are downstream. As water downstream has nowhere to go, it gets backed up in the upstream drainage of the St. Joe River, the Coeur d’Alene River and the St. Maries River, along with the many smaller streams.
You can access water flow information at myavista.com/waterflow or, if you prefer, the information is available on a 24-hour telephone information line. You will find notification of anticipated elevation changes on Lake Spokane, the Spokane River, and Coeur d’Alene Lake. In Washington call (509) 495-8043 and in Idaho call (208) 769-1357.
The recorded information is provided to advise shoreline property owners, commercial and recreational users of changes in the lake and river elevation levels that may affect plans for water use. And please remember to obey all warning signs around our dams.
Photo courtesy of DoMani Rae Di loli