Iraq is divided into three distinct zones: the desert in the west and southwest; the plains; and the highlands in the northeast, which rise to 3,000 m (10,000 ft) or more. The desert is an upland region with altitudes of 600 to 900 m (2,000–3,000 ft) between Damascus in Syria and Ar-Rutbah in Iraq, but declines gently toward the Euphrates (Al-Furat) River. The water supply comes from wells and wadis that at times carry torrential floods and that retain the winter rains.
Dominated by the river systems of the Tigris (Dijlah) and Euphrates, the plains area is composed of two regions divided by a ridge, some 75 m (250 ft) above the flood plain, between Ar-Ramadi and a point south of Baghdad that marks the prehistoric coastline of the Persian Gulf. The lower valley, built up by the silt the two rivers carry, consists of marshland, crisscrossed by drainage channels. At Qarmat 'Ali, just above Basra, the two rivers combine and form the Shatt al-'Arab, a broad waterway separating Iraq and Iran. The sources of the Euphrates and Tigris are in the Armenian Plateau. The Euphrates receives its main tributaries before entering Iraq, while the Tigris receives several streams on the eastern bank within the country.