Mesopotamia encompasses the land between the Euphrates and Tigris rivers, both of which have their headwaters in the Taurus Mountains. Both rivers are fed by numerous tributaries, and the entire river system drains a vast mountainous region. Overland routes in Mesopotamia usually follow the Euphrates because the banks of the Tigris are frequently steep and difficult. The climate of the region is semi-arid with a vast desert expanse in the north which gives way to a 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.