Brigham Young hears a rumour from some Mormons come up from Missouri, that John Fremont, the Pathfinder is now married to the daughter of the expansionist Senator, Thomas Hart Benton, and that Benton is pushing for acquisition of California. And that Fremont is surveying the Oregon Trail to create a guide for settlers aiming to cross the continent. That is enough to convince Brigham Young that his initial plan to settle in northernmost Mexico, near the Great Salt Lake is likely to leave the Saints ass deep in Gentiles.
But Vancouver Island is vulnerable because it is on the ocean and therefore under the power of the Royal Navy, which can enforce Great Britain's laws against polygamy. The Saints need a home that is inland, yet north of of the 49th parallel, reachable in one season and fertile. Brigham decides that the Saint's best bet is to ford the Snake River just past Marcus Whitman's Walla Walla Mission, proceed north by northwest from there until they reach the Columbia River and either follow the Columbia or cross the Columbia and follow another river north until past the 49th parallel.
And this is what they do, reaching Ft. Hall, following the Snake River to Ft. Boise than north over Baker Summit to the Whitman Mission, crossing the Snake River and the grassy but water short Columbia Basin until they reach the Columbia River near Lake Chelan. They cross the Columbia and then follow it north to the Okanogan River than follow the Okanogan River to Okanogan Lake. And there, they found their colony, soon discovering gold at Kamloops. They also discover that there are large patches of arable land all through inland north Oregon, as well as inlets where ships carrying converts from the UK and Scandinavia can disembark unobtrusively, and that if they get far enough northwest, they are in Russian Alaska.