Animas River (ON-ee-mas) is a 126-mile-long (203 km) river in the western United States, a tributary of the San Juan River, part of the Colorado River System.The river's free-flowing status ended when the Animas-La Plata Water Project was completed in 2015. The project pumps water over a low pass to fill a reservoir, Lake Nighthorse, in Ridges Basin to satisfy Southern Ute tribal water rights claims associated with the Colorado Ute Settlement Act amendments of 2000.