The valleys of the Debet and Akstafa rivers form the chief routes into Armenia from the north as they pass through the mountains. Lake Sevan, 72.5 kilometers across at its widest point and 376 kilometers long, is by far the largest lake. It lies 2,070 meters above sea level on the plateau. Terrain is most rugged in the extreme southeast, which is drained by the Bargushat River, and most moderate in the Aras River valley to the extreme southwest. Most of Armenia is drained by the Aras or its tributary, the Razdan, which flows from Lake Sevan. The Aras forms most of Armenia's border with Turkey and Iran as well as the border between Azerbaijan's adjacent Nakhichevan Autonomous Republic and Iran.