After the general meeting in 1806, Thompson travelled to Rocky Mountain House and prepared for an expedition to follow the Columbia River to the Pacific. In June 1807 Thompson crossed the Rocky Mountains and spent the summer surveying the Columbia basin; he continued to survey the area over the next few seasons.:38-65 Thompson mapped and established trading posts in Northwestern Montana, Idaho, Washington, and Western Canada. Trading posts he founded included Kootenae House, Kullyspell House and Saleesh House; the latter two were the first trading posts west of the Rockies in Idaho and Montana, respectively. These posts established by Thompson extended North West Company fur trading territory into the Columbia Basin drainage area. The maps he made of the Columbia River basin east of the Cascade Mountains were of such high quality and detail that they continued to be useful into the 20th-century:258.