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Watershed

The 534,769 square kilometres (206,475 sq mi) of the Amu Darya drainage basin include most of Tajikistan, the southwest corner of Kyrgyzstan, the northeast corner of Afghanistan, a narrow portion of eastern Turkmenistan and the western half of Uzbekistan. Part of the Amu Darya basin divide in Tajikistan forms that country's border with China (in the east) and Pakistan (to the south). About 61% of the drainage lies within Tajikistan, Uzbekistan and Turkmenistan, while 39% is in Afghanistan.

The abundant water flowing in the Amu Darya comes almost entirely from glaciers in the Pamir Mountains and Tian Shan, which, standing above the surrounding arid plain, collect atmospheric moisture which otherwise would probably escape elsewhere. Without its mountain water sources, the Amu Darya would not exist—because it rarely rains in the lowlands through which most of the river flows. Of the total drainage area, only about 200,000 square kilometres (77,000 sq mi) actively contribute water to the river. This is because many of the river's major tributaries (especially the Zeravshan River) have been diverted, and much of the river's drainage is arid. Throughout most of the steppe, the annual rainfall is about 300 millimetres (12 in).