10 May 2018 — 1848 mdt
Flathead River’s hydrograph at Blankenship
resembles hydrograph of flood year 1947
The Flathead River’s unfettered north and middle forks merge at the southwest tip of Glacier National Park just above the Blankenship Bridge, a popular recreational destination. A few miles downstream, just below Hungry Horse, at the eastern end of Badrock Canyon, the south fork joins the river to form the mainstem Flathead River.
The confluence of the north and middle forks of the Flathead River and the Blankenship Bridge. Larger image displaying the confluence of the north and middle forks, and the merging of the south fork at the entrance to Badrock Canyon (opens in new window).
Because the north and middle forks are wild flowing rivers, and the south fork is regulated by Hungry Horse Dam, which applied its chokehold at summer’s end in 1951, the river at Blankenship reveals the true runoff and serves as a proxy for the unregulated Flathead River above Flathead Lake.
Today, the river at Blankenship is rising, and moving as much water downstream as it did 71 years ago, in 1947.
In 1947, the mainstem at Columbia Falls peaked on 10 May at 77,400 cubic feet per second. Today, because HHD is holding back the south fork, releasing only 2,210 cfs, instead of the 30,500 cfs that coursed down the south fork in 1947, the river is discharging only ≈ 49,000 cfs and running approximately nine inches above the flood stage of 13 feet.
In the graph above, the horizontal distance between the blue 1947 hydrograph and the red 2018 hydrograph displays how HHD reduces flooding south of Columbia Falls.