In some respects the most important of the early Colorado River exploration journals, the diaries of Almon Harris Thompson can naturally be divided into three sections: navigation of the Green and Colorado rivers; exploratory traverse from Kanab to the mouth of the Fremont River; and the systematic mapping of central, eastern, and southern Utah and northern Arizona. Thompson's maps of the Colorado drainage basin, including the first maps of southern Utah and the canyons of the Green and Colorado rivers, place him in the front rank of geographic explorers and they proved invaluable to the later Powell expedition. Originally published in 1939 as volume seven of the Utah Historical Quarterly, Thompson's journal is reprinted here for the first time in seventy years. Co-published with the Utah State Historical Society.