It rises from snowmelt along the continental divide west of Conejos Peak in western Conejos County, approximately northeast of Pagosa Springs. It flows briefly northeast, through Platoro Reservoir, then southeast through the Rio Grande National Forest, then east along the New Mexico border through a scenic canyon. It enters the southwestern corner of the San Luis Valley from the west near Conejos and joins the Rio Grande from the west approximately 15 mi (24 km) southeast of Alamosa. It is impounded at Platoro Reservoir for flood control and to manage irrigation in the San Luis Valley, as part of the San Luis Valley Project of the U.S. Bureau of Reclamation.