the soviet union became the ruling power in the 20th century. the soviet union fell in the 1990s and central asia split up into the many smaller countries that lie within or partially within the amu darya basin. in the 1960s and 1970s, the amu darya and syr darya were first used by the soviets to irrigate extensive cotton fields in the central asian plain. before this time, water from the rivers was already being used for agriculture, but not on this massive scale. the qaraqum canal, karshi canal, and bukhara canal were among the larger of the irrigation diversions built. the main turkmen canal was a proposed project that would have diverted water along the dry uzboy river bed into central turkmenistan, but was never built.