A ribbon of green in an arid land, the 126-mile Animas River brings reliable year-round water to the Four Corners region. Starting in the San Juan Mountains, the Animas joins the San Juan River just south of here at Farmington. The Animas Glacier likely advanced for the first time 800,000 years ago and retreated for the last time about 12,000 years ago, leaving behind debris such as the round stone cobbles in the river channel today. The water and ecosystems of the Animas River sustained Ancestral Pueblo people in this area for centuries, and residents left the large pueblo settlement here sometime between the years 1280 and 1300. The Animas River is part of the vast Colorado River watershed, which spans nearly 250,000 square miles.