Complex sentence: Thompson mapped and established trading posts in Northwestern Montana, Idaho, Washington, and Western Canada; including Kootenae House, Kullyspell House and Saleesh House; the latter two of which were the first trading posts west of the Rockies in Idaho and Montana, respectively.
Split into simpler sentences: 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 of which were the first trading posts west of the Rockies in Idaho and Montana, respectively.