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Definition of alluvial fan

A low, outspread, gently sloping mass of loose rock material, shaped in plan view like an open fan or a segment of a cone; deposited by a stream (esp. in a semiarid region) at the place where it issues from a narrow mountain valley upon a plain or broad valley, or where a tributary stream is near or at its junction with the main stream, or wherever a constriction in a valley abruptly ceases or the gradient of the stream suddenly decreases; it is steepest near the mouth of the valley where its apex points upstream, and it slopes gently and convexly outward with gradually decreasing gradient.

Synonym of: fan, detrital fan, talus fan, dry delta

Compare with: alluvial cone, bajada

Ref: AGI
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