As a mountain stream moves into a flat area, it slows down and loses its capacity to carry as much alluvium as it did upstream, depositing the excess in sandbars throughout the channels. Over time, the channel migrates back and forth, creating fan-shaped deposits known as alluvial fans.
The narrowest point of an alluvial fan—closest to the mountain front—is known as the apex; the broader part is called the apron. Alluvium deposited closer to the apex tends to be coarser than the material that makes up the apron. Alluvial fans are more likely to form in deserts because there is plenty of loose alluvium and not much vegetation to prevent stream channels from shifting.
Alluvial fans in arid areas are often used for agriculture because they are relatively flat and provide groundwater for irrigation. This fan is no exception. The blocky green pattern across the apron indicates fields or pasture land. A number of towns and villages, including Usharal and Beskol, are visible along the fan’s outer edge. The straight feature cutting through Beskol and along the northeastern portion of the fan is railroad tracks.
Image courtesy of NASA.