The major lowland regions are the Andalusian Plain in the southwest, the Ebro Basin in the northeast, and the coastal plains. The Andalusian Plain is essentially a wide river valley through which the Río Guadalquivir flows. The river broadens out along its course, reaching its widest point at the Golfo de Cadiz. The Andalusian Plain is bounded on the north by the Sierra Morena and on the south by the Sistema Penibético; it narrows to an apex in the east where these two mountain chains meet. The Ebro Basin is formed by the Río Ebro valley, contained by mountains on three sides—the Sistema Ibérico to the south and west, the Pyrenees to the north and east, and their coastal extensions paralleling the shore to the east. Minor low-lying river valleys close to the Portuguese border are located on the Tagus and the Río Guadiana.