It's a beautiful photo but the title may be a bit confusing. This is actually the Flathead Valley, the photo taken from Elmo on the west shore of Flathead Lake.
The southern portion of this large mountain trench is generally considered "The Mission Valley", one of the more spectacular valleys anywhere. The northern portion is more generally known as the Flathead Valley as it encompasses the north end of the Mission Range as well as being bordered by the Swan range beyond. The northern part of the valley further divides into a number of other named valleys formed by geological overthrusting and glaciation.
Interestering. I must ask where is the dividing line between the Flathead and Mission Vallies? My photos are archived with all photos of the Valley and the Mission Mts under"Mission Vally", not meaning to be geologically correct but rather to define the general area where the photos were taken. The Flathead/Mission vallies area has some of the most interesting geologic history of any place in the US, well worth any effort to study it. Stumblingbear