The California gold rush of 1849 was the motivation for the next party to explore the Green River. William Manly and several companions entered the Green near the Sweetwater crossing, and floated in an abandoned ferryboat and later dugout canoes all the way to the Uinta Basin. Above the mouth of the White River, Manly met Wakara, chief of the Utes, who convinced the '49ers that the Green was not the easiest route to California, as they had thought. Manly and his party left the river and journeyed overland to Salt Lake City. The Mormons, who settled Salt Lake in 1847, sent exploring parties into the Uinta Basin as early as the 1850s, but the surveyors returned with unfavorable reports, and the basin of the Green remained unsettled by the Latter-day Saints for another twenty years. In the meantime, the Green River basin was acquired by the United States from Mexico through the Treaty of Guadelupe-Hidalgo in 1848.
It was not until 1869 that the Green was surveyed and mapped by a scientific party. John Wesley Powell embarked on the first of two voyages down the Green in May 1869 and floated the river all the way to its confluence with the Colorado and beyond. Powell left a detailed account of the river and the surrounding landscape and prepared the first thorough maps of the river basin. Powell left his mark on the in other ways as well. He and his men named most of the canyons, geographic features, and rapids along the Green River during his two voyages in 1869 and 1871. Powell also paved the way for later generations of explorers and scientists interested in the unique geology of the basin of the Green River.