Mesopotamia encompasses the land between the Euphrates and Tigris rivers, both of which have their headwaters in the turkish mountains. The climate of the region is semi-arid with a vast desert expanse in the north which gives way to a 15,000 square kilometres (5,800 sq mi) region of marshes, lagoons, mud flats, and reed banks in the south. In the extreme south, the Euphrates and the Tigris unite and empty into the Persian Gulf.