Syr Darya or Syrdarya (both: sēr däryäˈ, –därˈyə) [key], ancient Jaxartes or Yaxartes, Pers. Sihun, river, c.1,380 mi (2,220 km) long, flowing through Uzbekistan, Tajikistan, and Kazakhstan. One of the principal rivers of central Asia, it is formed in the Fergana Valley, E Uzbekistan, by the junction of the Naryn and Kara Darya rivers, which rise in the Tian Shan mts. It flows W through Tajikistan, then NW through Uzbekistan and Kazakhstan, past Kyzylorda, and into the northern basin of the Aral Sea. Its shallowness makes it unfit for navigation. Its waters are used for irrigating the important cotton-growing areas along its course and for hydroelectric power, but the river has become increasingly polluted, especially reaches Kazakhstan. The Syr Darya forms the northern and eastern limits of the Kyzyl Kum desert. It is paralleled in its lower course by the Trans-Caspian RR. Alexander the Great in his conquest of Persia reached the river c.329 B.C. and may have founded the chief city on its course—Khudjand—on the site of an older city.