What Is an Alluvial Fan? Where a Stream Leaves the Mountains and Drops Everything

Water flowing through a steep confined valley moves fast and carries a substantial load of sediment, including material far coarser than a gentle river can move. Where the valley opens onto flatter ground, the flow is no longer confined, so it spreads laterally, the depth drops, the velocity falls and the sediment is deposited, with the coarsest material dropping first near the apex and progressively finer material carried further out. That produces a cone radiating from the point where the valley opens, steepest at the apex and flattening outward. The channel then shifts, since deposition raises its own bed until the water finds a lower route across the fan, and repeating that over long periods distributes sediment across the whole surface and is what makes it a fan rather than a single ridge.

The size and activity of a fan depend on conditions in the catchment above:

• •Steep relief, which supplies both energy and the coarse material to move
• •Rock that weathers into loose debris rather than resisting erosion
• •A climate with intense infrequent rainfall, which produces the flash flows that move most of the material
• •Sparse vegetation, which fails to hold the slopes together
• •Tectonic activity, since ongoing uplift maintains the relief and faulting supplies broken rock
• •Recent glaciation or wildfire, both of which leave enormous quantities of loose material available

These surfaces attract settlement for reasons that are entirely sensible in ordinary conditions. They are flat compared with the mountains behind and elevated above the valley floor, which puts them above ordinary river flooding. The sediment is well drained and frequently fertile. Water is available where the stream emerges and frequently underground within the fan itself, which functions as an aquifer. The position at the mountain front is where routes converge. Cities in arid regions worldwide sit on them, with substantial urban areas in the western United States, the Middle East and Central Asia built across fan surfaces. The arrangement works well until the process that built the fan resumes, which it does at intervals far longer than a human lifetime and far shorter than the age of the landform.

Fans are recognisable once the shape is known and several features identify them. The overall form is a segment of a cone radiating from a notch in the mountain front, visible in plan on a map and in profile as a slope that steepens towards the apex. Vegetation frequently differs from the surrounding plain because drainage does. Older fan surfaces develop a dark desert varnish and a stony pavement while active parts of the surface are pale and loose, so the pattern of light and dark across a fan maps which parts have been active recently. Abandoned channels are visible as shallow braided traces. Where a fault runs along the mountain front, fans are offset and cut, which makes them useful recorders of movement, and dating fan surfaces is a standard method for establishing how fast a fault has moved.

Flooding on a fan behaves unlike river flooding and the difference is the danger. A river floods within a known valley and the areas at risk can be mapped from the channel. On a fan the channel itself moves, so a flow may take an entirely new route across ground that has no channel and no history of flooding within memory, which means the location of the hazard is uncertain in a way that conventional mapping does not capture. The flows are frequently debris flows rather than water, carrying a slurry of mud and boulders that moves fast, hits hard and buries what it reaches, and such flows have destroyed developments built on fans repeatedly. Wildfire above a fan sharply increases the risk for several years afterwards by removing vegetation and making slopes water-repellent, which is a recognised sequence that has produced fatal events.

A confined stream leaving the mountains spreads, slows and drops its load, building a cone that is coarsest at the apex, and the channel shifts as deposition raises its own bed. Flat ground, drainage, fertility and water make these attractive to settle. The channel moves, so a flow can take ground with no flooding history, and debris flows after wildfire are a documented and fatal sequence.