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Extending seasonal discharge records for streamgage sites on the North Fork Fortymile and Middle Fork Fortymile Rivers, Alaska, through water year 2019

January 31, 2020

Daily mean discharge values were estimated for May 20–September 30 for 1976–82 and 2006–18 for the U.S. Geological Survey North Fork Fortymile River and Middle Fork Fortymile River streamgage sites in Alaska. A relation between study streamgage discharge and discharge for an index streamgage on the main-stem Fortymile River for a concurrent period in 2019 was developed using the maintenance of variance extension type 3 (MOVE.3) record extension technique. The relation for North Fork Fortymile River discharges incorporated a 1-day-earlier offset to index streamgage discharges. No offset was applied to the index streamgage discharges for use with the Middle Fork Fortymile River discharges. The developed MOVE.3 regressions were used to estimate daily mean discharges at the study streamgage sites during the study season for the longer period of record of the index streamgage. The modified Nash-Sutcliffe efficiency coefficients for the estimated records were 0.38 and 0.63 for the North Fork Fortymile River and Middle Fork Fortymile River streamgages, respectively.