Watershed
About 1,385,045 square kilometres (534,769 sq mi) of land is drained by the Amu Darya into the Aral Sea endorheic basin. This includes most of Tajikistan, the southwest corner of Kyrgyzstan, the northeast corner of Afghanistan, a long narrow portion of western Turkmenistan and about half of Uzbekistan. Part of the Amu Darya's drainage 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. Of the area drained by the Amu Darya, 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 dominated by outlying desert and steppe.
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 somewhere else. Without its mountain water sources, the Amu Darya would not contain any water -- would not exist -- because it rarely rains in the lowlands through which most of the river flows. Throughout most of the steppe, the annual rainfall is about 300 millimetres (12 in).
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