He also sent a party of men under Joe Walker to explore the Great Salt Lake and to find an overland route to California. Walker discovered a route along the Humboldt River across present-day Nevada, as well as Walker Pass across the Sierra Nevada. The path later became known as the California Trail, the primary route for the immigrants to the gold fields during the California Gold Rush. Much speculation has surrounded Bonneville's motivation for sending Walker to California. Some historians have speculated that Bonneville was attempting to lay the groundwork for an eventual invasion of California, then part of Mexico, by the United States Army.