On May 3, 1853, he received orders to take charge of an expedition to survey a route for a Pacific railroad between the 38th and 39th parallels. The surveying party left St. Louis, Missouri, in June 1853 and arrived by mid-October in Manti, Utah Territory. In Utah Territory, with Lieutenant E. G. Beckwith as assistant commander, Gunnison began the survey of a possible route, surveying areas across the Rocky Mountains via the Huerfano River, through Cochetopa Pass, and by way of the present Gunnison and Green rivers to the Sevier River. His journey took him through the Tomichi Valley in Colorado, where the town of Gunnison is named in his honor. After crossing the Tomichi Valley, the survey team encountered the Black Canyon, carved by the Gunnison River which was also named in his honor. The team was forced to turn south to get around the canyon.