In 1845, Carson guided Frémont on their third expedition (there would be a fourth but without Carson). From Westport Landing, Missouri, they crossed the Rockies, passed the Great Salt Lake, and down the Humboldt River to the Sierra Nevada of California and Oregon. Frémont made scientific plans, included artist Edward Kern to his corps, but from the outset the expedition appeared to be political in nature. Frémont may have been working under secret government orders since US President Polk wanted Alta California for the United States. Once in California, Frémont started to rouse the American settlers into a patriotic fervor. The Mexican General Jose Castro at Monterey ordered him to leave. On Gavilan Mountain Frémont erected a makeshift fort and raised the American Flag in defiance before departing north. The party moved into the Sacramento River Valley past Mount Shasta, surveying into Oregon, fighting Indians along the way, and camped near Klamath Lake. Near here, a messenger from Washington, DC, caught up with Fremont and made it clear that Polk wanted California.