Where would you expect to find an alluvial fan?

Question

BlackTom AI · Accepted Answer

To approach this question, let's evaluate where an alluvial fan typically forms and then assess each option.
Option a: 'At the inner edge of a channel' – The inner edge of a channel is where sediment is often deposited in point bars within meandering sections, not where large graded fans form. This location does not correspond to the broad, fan-shaped deposit created by a sudden decrease in slope.
Option b: 'In a meander channel' – Meander channels are characterized by sinuous paths within floodplains. While sediment is moved and deposited locally, an alluvial fan is not formed inside a meander channel itself; fans require a rapid change in gradient typically outside the channel system.
Option c: 'At a river mouth' – River mouths lead to deltas in low-energy environments, where deposition produces layered sediments spreading into a standing body of water. An alluvial fan, however, forms where there is a drop in energy away from a highland area, not where a river enters a standing body of water in many cases.
Option d: 'At a canyon mouth' – This is the correct context. Alluvial fans commonly develop where streams exit narrow mountainous or upland regions into flatter terrain, such as at the mouth of a canyon. The abrupt wideness and decrease in gradient cause the heavier sediments to drop out, creating a fan-shaped deposit.
In summary, the distinguishing feature of an alluvial fan is deposition of coarse material at the downslope transition from confined, high-energy flow in canyons to more open, lower-energy environments, which aligns with option d. The other options describe settings where alternative fluvial or deltaic processes dominate and are not typical environments for alluvial fan formation.

SC/NATS 1750 A - The Earth and Its Atmosphere (Full Year 2024-2025)

Where would you expect to find an alluvial fan?

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