According to the Section Bylaws, the Columbia Valley Section consists of:
Eastern Washington and Northern Idaho. This area includes all of Okanogan, Chelan, Kittitas, Yakima, and Klickitat Counties, and all Washington Counties eastward to the Idaho border as well as all counties in Idaho north of and...
including Idaho County.
This is shown approximately in the following map where the colored areas represent the Section within Washington and Idaho. The total area is estimated at approximately 51,000 square miles (132,000 square km). Activities tend to be centered around Richland, Washington.