Where a steep mountain stream reaches flatter ground, its water slows and drops its load. An alluvial fan is the resulting fan-shaped deposit of sand, gravel, and boulders that spreads outward from the canyon mouth. The coarser material settles first near the apex; finer sediment travels farther.

Fans are common in arid and semi-arid regions where flash floods carry heavy sediment loads. Over time successive deposits build a gently sloping cone. Channels on the fan surface shift frequently, creating a braided pattern. In wetter climates vegetation may stabilize older surfaces.

Engineers watch alluvial fans closely because debris flows and sudden flooding can endanger roads and settlements built on them. Geologists read the internal layering to reconstruct past flood magnitudes and climate shifts. The form itself remains a clear signature of the transition from confined flow to unconfined deposition.

• Fan-shaped sediment deposit at a canyon mouth
• Forms when a stream loses velocity
• Common in dry mountainous regions
• Hazardous for construction due to debris flows