The first decent map of California and Oregon were drawn by Captain John C. Frémont of the U.S. Army's Corps of Topographical Engineers, and his topographers and cartographers in about 1848. Fremont and his men, led by his guide and former trapper Kit Carson, made extensive expeditions starting in 1844 over parts of California and Oregon including the important Humboldt River and Old Spanish Trail routes. They made numerous topographical measurements of longitude, latitude and elevation as well as cartographic sketches of the observable surroundings. His map, although in error in minor ways, was the best map available in 1848. John C. Frémont gave the Great Salt Lake, Humboldt River, Pyramid Lake, Carson River, Walker River, Old Spanish Trail etc. their current names. The Truckee River (called the Salmon-Trout River by Fremont) in California and Nevada was mapped. Lake Tahoe is shown but left unnamed. The major rivers in California are shown, presumably given the names used by the trappers and Mexican and foreign settlers there. The Humboldt was named (after the great explorer Alexander von Humboldt). Fremont and his topographers/cartographers did not have time (it would take literally decades of work to do this) to make extensive explorations of the entire Sierra Nevada range or Great Basin. Details of the Sierra Nevada and Great Basin concerning the best passes or possible emigrant routes for wagons would be explored and discovered from about 1846 to 1859 by numerous other explorers.