dry fan

geology

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Last updated

June 25, 2026

Quick Summary

An alluvial fan is a geological deposit that forms at the mouth of a mountain canyon where a stream's sediment transport slows or stops. These fan-shaped deposits are common in arid and semiarid regions and are characteristic landforms of deserts.1

Alluvial fans are important for groundwater resources in dry areas and can support entire cities. They are formed by sediment eroded from mountains and deposited in adjacent basins, often through flash floods. The material deposited ranges from coarse at the apex (near the canyon mouth) to finer sands and silts at the base.1

While often associated with dry climates ("dry fans"), alluvial fans can form in various climates ("wet fans") where streams flow constantly. Their size is influenced by factors like the source area's geology and regional climate.2

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river
Alluvial fans
…being one of two types—either dry or wet. Dry fans are those that seem to form under conditions of ephemeral flow, while wet fans are those that are created by streams that flow constantly. This classification suggests that fan type is climatically controlled, because ephemeral flow is normally associated with…
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…being one of two types—either dry or wet. Dry fans are those that seem to form under conditions of ephemeral flow, while wet fans are those that are created by streams that flow constantly. This classification suggests that fan type is climatically controlled, because ephemeral flow is normally associated with…
Read More