27 Facts About Alluvial fans

Alluvial fans are common in the geologic record, such as in the Triassic basins of eastern North America and the New Red Sandstone of south Devon.

Alluvial fans have been found on Mars and Titan, showing that fluvial processes have occurred on other worlds.

Some largest alluvial fans are found along the Himalaya mountain front on the Indo-Gangetic plain.

Alluvial fans vary greatly in size, from only a few meters across at the base to as much as 150 kilometers across, with a slope of 1.

Unconfined alluvial fans allow sediments to naturally fan out, and the shape of the fan is not influenced by other topological features.

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Debris flow Alluvial fans receive most of their sediments in the form of debris flows.

Debris flow Alluvial fans occur in all climates but are more common where the source rock is mudstone or matrix-rich saprolite rather than coarser, more permeable regolith.

Debris flow Alluvial fans have a network of mostly inactive distributary channels in the upper fan that gives way to mid- to lower-level lobes.

Fluvial Alluvial fans receive most of their sediments in the form of stream flow rather than debris flows.

Fluvial Alluvial fans occur where there is perennial, seasonal, or ephemeral stream flow that feeds a system of distributary channels on the fan.

Such alluvial fans tend to have a shallower slope but can become enormous.

Examples of paleoAlluvial fans include the Triassic basins of eastern North America and the New Red Sandstone of south Devon, the Devonian Hornelen Basin of Norway, and the Devonian-Carboniferous in the Gaspe Peninsula of Canada.

Alluvial fans are characterized by coarse sedimentation, though the sediments making up the fan become less coarse further from the apex.

However, a few Alluvial fans show normal grading indicating inactivity or even fan retreat, so that increasingly fine sediments are deposited on earlier coarser sediments.

Depositional facies that have been reported for alluvial fans include debris flows, sheet floods and upper regime stream floods, sieve deposits, and braided stream flows, each leaving their own characteristic sediment deposits that can be identified by geologists.

The medial fan of a streamflow-dominated alluvial fan shows nearly the same depositional facies as ordinary fluvial environments, so that identification of ancient alluvial fans must be based on radial paleomorphology in a piedmont setting.

Alluvial fans are characteristic of mountainous terrain in arid to semiarid climates, but are found in more humid environments subject to intense rainfall and in areas of modern glaciation.

Alluvial fans are built in response to erosion induced by tectonic uplift.

The younger Alluvial fans, in turn, are cut by deep incised valleys showing two terrace levels.

Alluvial fans are often found in desert areas, which are subjected to periodic flash floods from nearby thunderstorms in local hills.

Alluvial fans develop in wetter climates when high-relief terrain is located adjacent to low-relief terrain.

The crater rim alluvial fans appear to have been deposited by sheetflow rather than debris flows.

Alluvial fans have been observed by the Cassini-Huygens mission on Titan using the Cassini orbiter's synthetic aperture radar instrument.

Alluvial fans are the most important groundwater reservoirs in many regions.

However, flooding on alluvial fans poses unique problems for disaster prevention and preparation.

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Alluvial fans are subject to infrequent but often very damaging flooding, whose unusual characteristics distinguish alluvial fan floods from ordinary riverbank flooding.

Debris flow Alluvial fans make poor petroleum reservoirs, but fluvial Alluvial fans are potentially significant reservoirs.