Sorry for the late response, but my wife and I were away for a few weeks.
Suggested name(s) for our area:
1) Upper Columbia Basin
2) Columbia Basin
3) Upper Columbia
Columbia Lake represents the beginning, and highest point, along the entire course of the Columbia River.
There are many “valleys” along the course of the river before it empties into the Pacific Ocean near Astoria, Washington.
A term such as “upper” distinguishes it as being special from all other valleys along the entire route.
Radium Hot Springs