The San Juan River begins in Archuleta County, Colorado at the confluence of its East and West Forks. Both forks originate above elevations of in the eastern San Juan Mountains in the San Juan National Forest. The river flows southwest through the foothills of the Rocky Mountains through the town of Pagosa Springs and reaches the Navajo Lake reservoir just north of the New Mexico border, near Arboles, Colorado. Below the Navajo Dam the San Juan River flows west through a narrow farming valley in high desert country of the Colorado Plateau. At Farmington, New Mexico, it is joined from the north by its main tributary, the Animas River, which rises in the San Juan Mountains near Silverton, Colorado.