Mountain streams are usually confined to narrow channels and tend to transport sizable amounts of gravel, sand, clay, and silt—material that geologists call alluvium. ... Over time, the channel migrates back and forth, creating fan-shaped deposits known as alluvial fans.

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Note added at 28 mins (2020-11-16 16:39:03 GMT)
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Если в тексте помимо горных потоков упоминаются другие их источники, то в препозиции можно добавить mountain в качестве определения (mountain alluvial fans).

https://link.springer.com/referenceworkentry/10.1007/3-540-3...

Alluvial fans are depositional features, formed of coarse gravel sediments, created where high-bed-load streams enter zones of reduced stream power, and deposit the coarser fraction of their loads. The resultant landforms are usually fan-shaped in plan and wedge-shaped in profile (Bull, 1977). They occur commonly in two topographic situations: at mountain fronts and at tributary junctions (Harvey, 1997). They are subaerial features, however if they extend into water they are known as fan-deltas.