Central Asia is bounded on the north by the forests of Siberia. The northern half of Central Asia (Kazakhstan) is the middle part of the Eurasian steppe. Westward the Kazakh steppe merges into the Russian-Ukrainian steppe and eastward into the steppes and deserts of Dzungaria and Mongolia. Southward the land becomes increasingly dry and the nomadic population increasingly thin. The south supports areas of dense population and cities wherever irrigation is possible. The main irrigated areas are along the eastern mountains, along the Oxus and Jaxartes Rivers and along the north flank of the Kopet Dagh near the Persian border. East of the Kopet Dagh is the important oasis of Merv and then a few places in Afghanistan like Herat and Balkh. Two projections of the Tian Shan create three bays along the eastern mountains. The largest, in the north, is eastern Kazakhstan, traditionally called Jetysu or Semirechye which contains Lake Balkhash. In the center is the small but densely-populated Ferghana valley. In the south is Bactria, later called Tocharistan, which is bounded on the south by the Hindu Kush mountains of Afghanistan. The Syr Darya (Jaxartes) rises in the Ferghana valley and the Amu Darya (Oxus) rises in Bactria. Both flow northwest into the Aral Sea. Where the Oxus meets the Aral Sea it forms a large delta called Khwarazm and later the Khanate of Khiva. North of the Oxus is the less-famous but equally important Zarafshan River which waters the great trading cities of Bokhara and Samarkand. The other great commercial city was Tashkent northwest of the mouth of the Ferghana valley. The land immediately north of the Oxus was called Transoxiana and also Sogdia, especially when referring to the Sogdian merchants who dominated the silk road trade.