Alluvial fan

An alluvial fan is a fan-shaped sediment deposit formed where a stream exits a steep, confined valley and spreads across flatter terrain. Its branching channels and sediment patterns reflect changes in water flow and grain size.

Alluvial processes: Processes by which running water erodes, transports, and deposits sediment. These processes build fans as streams leave confined valleys and lose transporting power.
Sediment transport: The movement of particles by water, wind, ice, or gravity. A fan records sediment transported from an upland catchment to a low-gradient surface.
Paleoflood hydrology: The study of ancient floods using geological and biological evidence. Fan layers can preserve deposits from floods that predate written records.
Death Valley alluvial fans: Fan-shaped sediment deposits along the mountain fronts of California’s Death Valley. Their desert setting makes channel forms and fan surfaces especially clear.
Debris flow: A rapid flow of water mixed with abundant sediment and larger clasts. Debris flows can carry coarse material onto a fan and deposit it abruptly.
Grain size: A measure of the diameter of individual sediment particles. Grain size often decreases outward as the flow spreads and loses energy.
Groundwater recharge: The movement of water from the surface into an aquifer. Coarse, permeable fan sediments can transmit water downward to aquifers.
Bajada: A continuous sloping apron formed where neighboring alluvial fans merge. Coalescing fans create the extensive piedmont aprons seen in arid basins.
Graded bedding: A sedimentary layer in which grain size changes systematically from bottom to top. Individual fan deposits may fine upward as a flow loses energy.
Braided river: A river with multiple shallow channels separated by temporary sediment bars. Braided channels often migrate across fans as sediment supply fluctuates.

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