A reality based independent journal of observation & analysis, serving the Flathead Valley & Montana since 2006. © James Conner.

29 June 2015

Hydrographs of historically dry Flathead springs

Yesterday’s official high in the Flathead was 102°F at Glacier Park International Airport (KGPI), but a private weather station near the airport reported 104°F. Forests and fields are bone dry. And local rivers are flowing at near historic lows.

Assessing the degree of drought from the discharge of the Flathead River at Columbia Falls is not a straightforward process. Since Fall, 1951, Hungry Horse Dam has regulated the flow of the South Fork, distorting the hydrograph of the mainstem. Therefore, it’s useful to consider just the unregulated North and Middle forks of the river, which join at Blankenship and are gaged approximately three miles upstream of their confluence. Good records for both gages date back to 1 October 1939. Because the sum of the two forks correlates highly with the mainstem prior to the closing of HHD, it can be used as a proxy for the unregulated mainstem.

Over the weekend I looked at the sum of the North and Middle forks for May and June for the length of the record, calculating the mean discharge for the 61-day period (in calculating the mean for 2015, I assumed that the last three days of the month would have the same discharge as 27 June). The peak discharge generally occurs around 1 June, with snowmelt the major contributor in May and rain the major contributor in June. May–June, 2015, probably will have the fourth lowest discharge since 1940, lower than 2001, lower than 1992:

Below, the March through September hydrographs for historically dry years, beginning with the driest year in the record, 1941: