Terrain is most rugged in the extreme southeast, which is drained by the Bargushat River, and most moderate in the Araks River valley to the extreme southwest. Most of Armenia is drained by the Araks or its tributary, the Hrazdan, which flows from Lake Sevan. The Araks forms most of Armenia's border with Turkey and Iran while the Zangezur Mountains form the border between Armenia's southern province of Syunik and Azerbaijan's adjacent Nakhchivan Autonomous Republic.