Alluvial Fans By Tim Baker & Tom Coburn

Basic Definition: An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens following a steep gradient, slows, and spreads out – typically at the exit of a canyon onto a flatter plain. A convergence of neighbouring alluvial fans that cover a huge area at the bottom of a slope is called a bajada, or compound alluvial fan.

Formation: Alluvial fans are fan-shaped landforms that form in areas where there is a sudden change in landscape, from a steep gradient (such as a mountainous region) to a flat plain. They are similar to deltas, but are deposited on land, rather than in the sea. A river flows down the slope, with gravity creating plenty of energy and meaning it can carry a large load.

Formation (continued): The river then flows out onto the flatter land and all the energy it had in its previous stage is suddenly diminished substantially. This dramatic loss of energy, results in deposition. At the apex of the fan, the larger, coarser material is deposited as this is the part of the load that requires the most energy. As you move further away from the apex, the deposited material is the finer sands and gravel.

Formation (continued) The deposition of material in the river channel chokes the flow and causes the water to look for another way to reach its destination. The resulting streams are called distributaries. This creates the fan shape, as the river continues to find a different route, and then the deposition of more material blocks it up, creating the cone-shaped fan that becomes broader and shallower further away from the apex.

An example of an alluvial fan forming in the mountains of Spitsberg, Norway. These are perfect conditions for the creation of alluvial fans.

Features: The shape of the fan is related to the grain size. Fans built of boulders and cobbles have a high pronounced arch, whose pavements may contain fragments a metre or more across that protrude well above the average surface Fans built of silt, sand, and fine gravel have broad, flattened profiles

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