In 1819, Major Stephen Harriman Long led a group of explorers towards the Rocky Mountains along the Platte River. Only a few Europeans had seen the river before, including Etienne de Veniard, Sieur de Bourgmont in 1714. The United States had just obtained the Platte in the Louisiana Purchase from France. Long's job was to explore and define the U.S.'s new border with Spanish lands to the south. From Council Bluffs, Iowa, the party followed the Platte's course westward until they reached a tall range that Long named the Rocky Mountains. From there, Long headed south. In the process, he spotted one of the tallest peaks in Colorado, which would later be named after him.

Long's map of the region shows the Boulder Valley as the western edge of a Great Desert. Delivering the map, he reported that lands here were "unfit for cultivation and of course uninhabitable by a people depending upon agriculture."  One interesting mistake Long made: he shows water west of Longs Peak flowing north into the Columbia River, rather than south into the Colorado drainage.

Stephen Long's map from his 1820 expedition is courtesy of the National Archives, Map # NWCS-077-CWMF-US62-sheet 1 of 2, 1820.