One of De Smet's longest explorations began in August 1845 in the region west of the Rockies that was jointly occupied by the Americans, who called it Oregon Country, and the British, who identified it as Columbia District. De Smet started from Lake Pend Oreille in present-day north Idaho and crossed into the Kootenay River Valley. He followed the Kootenay valley north, eventually crossing over to Columbia Lake, the source of the Columbia River at Canal Flats.