Flathead Lake’s outlet is fairly shallow. As displayed on the graphs below, the outflow from the lake does not reach 15,000 cfs — the approximate turbine capacity for Kerr Dam — until the lake rises to 2885.75 Somers Datum.
I generated the curves from data from the USGS river gage at Polson, and the lake gage at Somers, for 1 October 1929 through 30 September 1937, the most recent period before Kerr Dam began regulating outflow from the lake.