Surrounding Lake Amistad on the US-Mexico border, this recreation area is popular for water sports. The 85mi-long lake is formed by a dam built in 1969 below the confluence of the Pecos and Devils Rivers with the Rio Grande. Amistad, the Spanish word for "friendship," was a joint US-Mexico project. Much of the lake is lined with limestone canyons, some containing caves with prehistoric pictographs. There are more than 250 sites within 100sq mi.