If you wanted to go from Santa Fe to Los Angeles today, you’d hop on Interstate 4o and zoom across New Mexico, Arizona and southern California. This 830-mile journey whizzes past mesas, canyons, arroyos, mountains and gullies.

Travel back nearly two centuries, when tens of thousands of wayfarers went the long” way to California through the Western Slope. This Old Spanish Trail featured multiple routes for travelers, and three of them passed through Colorado. The northern branch went through G...