The supreme assignment of his army career was to lead an expedition in 1849 to the valley of the Great Salt Lake. His orders directed him to survey and map the Great Salt Lake and its valley as well as Utah Valley; to evaluate the various emigrant roads in the area, including the Oregon Trail; and to examine and report on the capability of the Mormon community at Salt Lake City to provide food and supplies for overland travelers. During the year he and his second in command, Lieutenant J.W. Gunnison, spent in Utah, Stansbury completed his assignment and produced a remarkable Report, which also went through several editions as a private publication. Stansbury's Report along with Gunnison's book, The Mormons, provided the outside world with an objective look at the Mormons of Utah as well as with a scientific appraisal of the resources and fauna and flora of this section of the Great Basin.