USGS Water-Year Summary 2024

12371550 FLATHEAD LAKE AT POLSON, MT

LOCATION - Lat 47°41'48.87", long 114°09'40.67" referenced to North American Datum of 1983, in SW 1/4 SE 1/4 NE 1/4 sec.04, T.22 N., R.20 W., Lake County, MT, Hydrologic Unit 17010208, at Polson.

DRAINAGE AREA - 7,086 mi².

SURFACE-WATER RECORDS

PERIOD OF RECORD - April to August 1900, daily lake elevations only, at site near Holt, 6 mi east of Somers (elevation unknown). August 1908 to November 1909 (fragmentary), January 1910 to Sept.30, 1998 published as "at Somers". Monthend contents only for some periods, published in Water Supply Paper 1316. Prior to April 1923, published as "at Polson." Oct. 1, 1941 to Sept. 30, 1998, unpublished daily lake elevations at Polson are available in files at the USGS Water Science Center located in Helena, Montana. October 1, 1998 to September 30, 2013, daily mean elevations and monthend contents. October 1, 2013 to current year, daily mean elevations.

GAGE - Water-stage recorder. Elevation of the gage is 2,878 ft., Somers Datum (2,877 ft., National Geodetic Vertical Datum of 1929). Subtract 1 foot to convert Somers datum to NGVD29. July 1 to December 12, 1923, nonrecording gage at Somers site.

REMARKS - Natural storage in Flathead Lake increased by construction of Kerr Dam 4 mi downstream from natural lake outlet; storage began Apr. 11, 1938. Usable capacity, 1,791,000 acre-ft at controlled spillway elevation 2,893.00 ft. Dead storage unknown below 2,878 ft, elevation of natural outlet. Minimum operating level, 572,300 acre-ft, elevation 2,883.00 ft for on-site power generation. Water is used for power production, flood control, recreation, and irrigation.

EXTREMES FOR PERIOD OF RECORD - Maximum elevation, 2,896.26 ft. on June 19, 1933; minimum elevation, 2,881.07 ft. on Dec. 5, 1936.