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River deltas and Lake Ayakum in China Tibet
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River deltas and Lake Ayakum in China (Tibet)

May 10, 2011
River deltas and Lake Ayakum in China (Tibet) are featured in this image photographed by an Expedition 27 crew member on the International Space Station. The Tibetan Plateau contains numerous lakes that dot an otherwise arid landscape. Lake Ayakum is located near the northern boundary of the Plateau to the southeast of the Kunlun Mountains. While many of the small glacier- and snowmelt-fed streams that cross the Tibetan Plateau eventually give rise to major Southeast Asian rivers including the Mekong and Yangtze, some empty into saline lakes such as Lake Ayakum. This detailed photograph highlights two river deltas (upper left and lower right) formed along the southwestern shoreline of the lake. When sediments build up to the point that a river can no longer flow over them, it will jump to a new channel position and begin the process anew. Scientists have noted that, over geologic time, the channels tend to sweep back and forth - similar to the motion of an automobile windshield wiper - to form the typical semi-circular or fan shape of the delta. Gray to tan surfaces of both deltas indicate prior positions of their respective river channels; the uniform coloration and smooth texture suggest that they are relatively old and are now inactive. In contrast, the younger and currently active delta surfaces can be recognized by reddish-brown sediment and clearly visible river channels. Lateral channel migration is particularly evident in the approximately eight-kilometer-wide active delta area at upper left. The reddish coloration of the actively depositing sediment may indicate a change from the sources that formed the older parts of the deltas (or indicate weathering and soil formation on the older deposits), or an episodic input of dust or other material to the river catchments.