Historic hydrographs of Flathead Lake. Before Kerr 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.

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.