Kootenay Lake is a long narrow and deep fjord-like lake. The basin has steep sides, a relatively flat bottom, a U-shaped cross-section profile and a very narrow littoral zone. At each end of the lake the bottom gradually shallows up to the deltas of the Kootenay and Duncan Rivers. Relatively gentle slopes are associated with the deltas of the larger streams that flow into the lake from its flanks but these deltas are of limited areal extent.

The geological origin of Kootenay Lake is said to be the result of river erosion which commenced in the late Cretaceous and continued through the Tertiary keeping pace with the growing mountain ranges until ice filled the valley in the Pleistocene. Valley glaciers originating in the Purcell and Selkirk Mountains fed the main ice mass from the sides. During this time the large valley glacier that occupied what is now the West Arm (outlet) of Kootenay Lake flowed into the Kootenay ice mass from the west. A large terminal moraine formed near what is now the big bend in the Kootenay River near Libby, Montana. As ice melted a lake formed behind the moraine and drained southward over it. As ice melted from the West Arm drainage became established over a lower divide near Nelson causing the West Arm to drain toward the west. The southerly drainage over the moraine dried up and the Kootenay River began to follow its present arcuate path (Fig. NAM-25-01).

The lake has long served as a transportation route. Native Indian populations first used the lake and associated river systems as part of their migration and trading routes. Mining activity began in the drainage basin with the discovery of gold in the 1820's near Bonners Ferry, Idaho. Although mineralization was known around the shores of Kootenay Lake as early as 1825, it was not until 1890 that wholesale staking and development began. For the most part the late development, as in the upper Kootenay Valley (Kimberley), was due to the late change in attitude of the miners from the search for 'easily won' placer deposits to the more laborious techniques of lode-mining. Mining activity in the Kootenay basin blossomed between 1905 and 1920, then collapsed during the depression years. Fruit growing was touted as a significant industry in the bench lands around the West and North Arms in the early 1900's; however a short growing season ended this dream. Fruit production has remained an important agricultural crop at the south end of the lake near Creston, B. C.

Today Kootenay Lake is a major recreation area for both local residents and tourists. A well known sports fishery, sailing and beautiful scenery draw many to the area.

Water quality in the lake was affected by the construction of a fertilizer plant in the early 1950's. Large loadings of phosphorus entered the Kootenay River drainage causing nuisance blue-green algae blooms in the 1950's, 1960's and early 1970's. Pollution abatement measures and subsequent closure of the plant, together with construction of the Libby Dam on the Kootenay River, have reduced phosphorus loadings to the lake to less than historic levels in 1988.

The overall impact of reductions to phosphorus loading to the lake and a subsequent decrease in algal productivity is not yet well understood. No discernible annual trends in zooplankton were evident in a 1984 study. A major decline in Kokanee stocks associated with the lake outlet may be in part related to the reduction of phosphorus to the lake (1).