Updated. The Montana Supreme Court’s decision in Mattson v. Montana Power, handed down on 25 August 2009, contains a possibly questionable number in its description of the annual hydrograph of Flathead Lake prior to the closing of Kerr Dam in the spring of 1938.
The case addressed whether Montana Power and PPL Montana (the current operator of the dam) acted outside the scope of “…easements obtained from shoreline property owners in the 1930s, ’40s, and ’50s, which allow the operator of Kerr Dam to flood, subirrigate, drain, or otherwise affect the Landowners’ properties with the waters of Flathead Lake.”
In the section of the decision “Additional Background,” which begins on Page 6 of the decision, the court writes:
Flathead Lake is fed by snowmelt and by releases from Hungry Horse Dam on the South Fork of the upper Flathead River (3). Prior to the construction of Kerr Dam, the lake’s water level rose an average of eight feet each year from mid-April to early June due to spring runoff. The average peak elevation was 2,890 feet above mean sea level. The water level then dropped steadily during the summer to a base level where it would remain until the following spring. Under Kerr Dam operations, however, the lake rises to an elevation some three to four feet above the average pre-dam peak and is maintained at this level into October. It is then lowered gradually over the winter to an elevation two or three feet above the pre-dam base level, after which spring runoff begins the cycle anew.
[Footnote]3 Hungry Horse Dam was completed in 1953. [JRC note. HHD began impounding water on 21 September 1951.]
The court’s value of 2,890 feet above mean sea level seems low, and may be the result of a failure to account for a 3-foot shift in the datum at the USGS’s gaging station at Polson on 1 October 1922. My review of the available data suggests that the average annual peak lake level prior to Kerr Dam was 2890.9 feet above mean sea level if just the peaks from the corrected published record at Polson are used. If calculated values(1) for 1908, 1923, 1924, 1925, and 1927, and the measurements at Somers(2) for 1928, are used, the mean annual peak increases to 2891.2 (plus or minus approximately slightly less than one-tenth-foot to take into account the uncertainty in the calculations).
The table below displays the annual pre-Kerr peaks for several data sets. For the best peak column, I used the corrected data from Polson; data from Somers if no data from Polson were available; calculated values if data from both lake gages were missing. In the table, the calculated values in the best values column are in blue italic, and the Somers measurement is in red. To obtain feet above mean sea level, subtract one foot from the numbers in the table (I know; it can get confusing).
Prior to 1 October 1921, elevations at Polson were obtained with a staff gage at the steamboat pier, so the error bars for this era are a bit wider than for water years 1922 and later.
| Date | Water Year | Best Peak Value | Polson Raw | Polson Corrected | Somers | Lake Calculated | Flathead River at Polson |
|---|---|---|---|---|---|---|---|
| Jun 12, 1908 | 1908 | 2894.05 | 2894.05 | 62,100 | |||
| Jun 21, 1909 | 1909 | 2893.20 | 2890.20 | 2893.20 | 2893.45 | 58,400 | |
| May 15, 1910 | 1910 | 2891.00 | 2888.00 | 2891.00 | 2890.78 | 42,800 | |
| Jun 18, 1911 | 1911 | 2891.40 | 2888.40 | 2891.40 | 2890.92 | 43,600 | |
| Jun 15, 1912 | 1912 | 2890.00 | 2887.00 | 2890.00 | 2889.69 | 36,500 | |
| Jun 10, 1913 | 1913 | 2895.50 | 2892.50 | 2895.50 | 2895.92 | 75,400 | |
| Jun 5, 1914 | 1914 | 2889.80 | 2886.80 | 2889.80 | 2889.36 | 34,600 | |
| May 17, 1915 | 1915 | 2887.00 | 2884.00 | 2887.00 | 2886.91 | 20,500 | |
| Jul 1, 1916 | 1916 | 2895.70 | 2892.70 | 2895.70 | 2895.84 | 74,700 | |
| Jun 23, 1917 | 1917 | 2893.50 | 2890.50 | 2893.50 | 2893.56 | 59,100 | |
| Jun 17, 1918 | 1918 | 2893.30 | 2890.30 | 2893.30 | 2893.21 | 57,000 | |
| Jun 1, 1919 | 1919 | 2891.80 | 2888.80 | 2891.80 | 2891.72 | 48,200 | |
| Jun 21, 1920 | 1920 | 2891.30 | 2888.30 | 2891.30 | 2890.92 | 43,600 | |
| Jun 10, 1921 | 1921 | 2893.70 | 2890.70 | 2893.70 | 2893.92 | 61,300 | |
| Jun 10, 1922 | 1922 | 2892.70 | 2889.70 | 2892.70 | 2893.11 | 56,400 | |
| Jun 14, 1923 | 1923 | 2892.96 | 2892.96 | 55,500 | |||
| May 22, 1924 | 1924 | 2891.34 | 2891.34 | 46,000 | |||
| May 31, 1925 | 1925 | 2894.70 | 2894.70 | 66,300 | |||
| May 8, 1926 | 1926 | 2888.45 | 2888.45 | 2888.45 | 2888.47 | 29,300 | |
| Jun 19, 1927 | 1927 | 2895.88 | 2895.88 | 75,000 | |||
| May 30, 1928 | 1928 | 2895.85 | 2895.85 | 2896.50 | 82,100 | ||
| Jun 15, 1929 | 1929 | 2890.61 | 2890.61 | 2890.61 | 2890.50 | 2891.09 | 44,600 |
| Jun 12, 1930 | 1930 | 2888.68 | 2888.68 | 2888.68 | 2888.66 | 2888.64 | 30,300 |
| May 19, 1931 | 1931 | 2889.16 | 2889.16 | 2889.16 | 2889.25 | 2889.38 | 34,700 |
| May 25, 1932 | 1932 | 2892.49 | 2892.49 | 2892.49 | 2892.58 | 2892.23 | 51,200 |
| Jun 19, 1933 | 1933 | 2896.15 | 2896.15 | 2896.15 | 2896.20 | 2896.05 | 76,600 |
| May 12, 1934 | 1934 | 2892.20 | 2892.20 | 2892.20 | 2892.20 | 2892.28 | 51,500 |
| Jun 5, 1935 | 1935 | 2891.91 | 2891.91 | 2891.91 | 2891.90 | 2891.94 | 49,500 |
| May 18, 1936 | 1936 | 2892.03 | 2892.03 | 2892.03 | 2892.10 | 2892.06 | 50,200 |
| Jun 7, 1937 | 1937 | 2889.54 | 2889.54 | 2889.54 | 2889.40 | 2889.36 | 34,600 |
________________________
Notes:
(1) The missing lake level values can be calculated from the discharge of the Flathead River at Polson, where the record of daily streamflow measurements is complete from August, 1907, through the present, except for a 6-month gap in the 1927 federal water year (federal water years begin on 1 October and end on 30 September of the following year). This is possible because the elevation of the lake and the discharge at Polson were highly correlated prior to Kerr Dam.
(2) The gage at Somers recorded slightly higher lake levels than the gage at Polson. I compared 3,176 pairs of lake levels from the 1930s. The differences ranged from -0.49 to 0.58 feet with a mean of 0.023 feet and an RMS of 0.11 feet. In a dead flat calm, all points on the lake are at the same elevation. Whether the differences are the results of heavy winds pushing the water to one end of the lake, instrument calibration errors, surveying errors, or something else, cannot be inferred from the record. I would not expect the gages to agree exactly very often. A difference of one-tenth-foot is inconsequential for the purposes of this analysis; indeed, for most practical purposes. Therefore, I consider the Polson and Somers values to be interchangeable for most purposes.