The Aral Sea, once one of the four largest lakes in the world, is located on the border of Kazakhstan and Uzbekistan . The two rivers that feed the lake, the Syr Darya and the Amu Darya, originate in the mountainous region to the east, primarily in the neighboring countries of Tajikistan, Kyrgyzstan and Afghanistan. Turkmenistan also relies on the waters of the Amu Darya, a portion of which is diverted into its borders via the Karakum Canal. Large-scale production of select crops, especially cotton, became a staple of the region under Soviet rule. The heavy use of the waters of the two rivers for crop irrigation reduced the flow of the rivers into the Aral Sea, reducing water levels starting in the 1960s and increasing salinity. By 1990, the lake had separated into two halves as the water levels dropped.