The portion of the State west of the Sierra Madre Oriental lies on the Mexican Plateau, and is part of the Chihuahuan Desert. The Bolsón de Mapimí is a large endorheic basin which covers much of the western portion of the State and extends into adjacent portions of Chihuahua, Durango, and Zacatecas. The Nazas River, which flows east from Durango, and the Aguanaval River, which flows north from Zacatecas, empty into lakes in the Bolsón. Torreón, the most populous city in the State, lies on the Nazas in the irrigated Laguna Region, the "(Comarca Lagunera)", which straddles the border of Coahuila and Durango.