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Why do many river valleys in the Takla Makan Desert become filled with sand?

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Map of Asia highlighting the location of the Takla Makan Desert in northwestern China, with the desert area shaded in orange. Surrounding regions, including parts of Europe, Africa, and the Pacific Ocean, are shown for geographic context. An inset in the top right corner provides a global view with the desert's location marked. — Takla Makan Desert The Takla Makan Desert is flanked by high mountain ranges: the Tien Shan to the north, the Kunlun Mountains to the south, and the Pamirs to the west.

In the Takla Makan Desert, many river valleys become filled with sand because the rivers themselves are lost to the desert environment. As rivers flow from the surrounding mountains into the desert, they gradually dry up within the sands. Once these rivers disappear, their empty valleys are subsequently filled by wind-borne sand.12

This process is driven by the desert's complex wind conditions and extreme aridity, which result in a thick cover of wind-blown sand across the region. While some rivers, such as the Hotan and Keriya, have survived, most of the shallower rivers have been unable to maintain their flow, leaving their former channels vulnerable to being reclaimed by the shifting eolian (wind-formed) landscape.31