In the 1990s, the U.S. Geological Survey, which collects data from thousands of gages on rivers and lakes, began publishing streamflow and lake level data on the internet. Streamflow records for the Flathead River below Polson begin in 1907 and, with one six-month gap in 1927–1928, extend unbroken through the present. Systematic measurements of Flathead Lake began a few years later, with many gaps, especially in the 1920s, until the 1930 federal water year (1 October 1929 to 30 September 1930), after which the record is unbroken through the present. Through 1998, there were gages at Polson and Somers. The average difference between the gages is approximately one-tenth-foot, and not significant from any practical standpoint. Using historical data, I calculated a lake stage/river discharge curve, from which I then was able to construct the hydrograph for the lake (except for several months in 1927–1928, a major flood year).
Update, 5 September 2015. Today, the Confederated Salish and Kootenai Tribes officially took possession of Kerr Dam and renamed it the Salish-Kootenai Dam. The new name won’t appear on maps and documents for months, and on historical documents not at all, so when writing about the dam, I’ll use its current name, but note that once it was known as Kerr Dam.
Annual hydrographs of Flathead Lake. Before the Salish-Kootenai Dam began regulating the level of Flathead Lake in the spring of 1938, the elevation of the lake and the discharge of the Flathead River below Polson were tightly correlated with each other. I was therefore able to use that relationship to reconstruct hydrographs for the lake during the periods before 1929 when the lake was not gaged. The elevations are all Somers Datum.
Outlet channel restrictions. The maximum hydraulic capacity of the Salish-Kootenai Dam’s powerhouse is approximately 15,000 cfs. When the lake drops below 2886.75, the discharge from the lake begins dropping below 15,000 cfs, reducing electrical generation. Using data from 1929–1937, I constructed a stage/discharge curve that displays the effect of Flathead lake levels on generation.
Major floods of the Flathead River. Major floods occurred in 1894, 1916, 1927, 1928, 1933, 1948, 1964, and 1975. The USGS inferred discharge and stage for 1894, evidently measured the peak value in 1928, and extrapolated the rating curve for the mainstem river at Columbia Falls upward in 1964, and upward at West Glacier for the Middle Fork in 1975. The magnitude of the 1927 flood can be inferred from the river’s discharge at Polson. In 1916, the North and Middle Forks were gaged. I derived a regression equation from data from the 1930’s. When I applied it to the values for 1916, it was obvious that the Flathead’s peak discharge at Columbia Falls reached at least 100,000 cfs, making it comparable to the floods of 1928 and 1948.
In progress, droughts of the Flathead Basin.