The Khabur River is the largest perennial tributary to the Euphrates in Syrian territory. Although the Khabur originates in Turkey, the karstic springs around Ra's al-'Ayn are the river's main source of water. Several important wadis join the Khabur north of Al-Hasakah, together creating what is known as the Khabur Triangle, or Upper Khabur area. From north to south, annual rainfall in the Khabur basin decreases from over 400 mm to less than 200 mm, making the river a vital water source for agriculture throughout history. The Khabur joins the Euphrates near the town of Busayrah.