David Thompson (1770-1857) is considered by many to have been the most important surveyor of North America. His achievements -- mapping the Saskatchewan River, the great bend of the Missouri River, the Great Lakes and the headwaters of the Mississippi as well as the Columbia watershed -- are the stuff of legend. Late in life Thompson wrote a retrospective memoir of his explorations, but the best way to understand his years in the fur trade is by reading his journals.

In her new Preface to the Bicentennial Edition of Columbia Journals, Barbara Belyea considers the fur-trade...