Helmand or Helmund (both: hĕlˈmənd) [key], river, c.700 mi (1,130 km) long, rising in the Hindu Kush Mts., NE Afghanistan and flowing generally SW to the Sistan basin, SW Afghanistan, where it helps form the Hamun-i-Helmand (Sistan Lake), a marshy lake that extends into Iran; longest river in Afghanistan. The Arghandab is its chief tributary. The Helmand's ancient irrigation and river-control system was destroyed by Jenghiz Khan (13th cent.) and Timur (14th cent.); the modern irrigation works are vital to both Iranians and Afghans, and in times of drought there are disputes over water rights. The Helmand Valley Authority extensively developed the region, improving irrigation and flood control. The name also occurs as Hilmand and Hirmand.