Geography · Ch 12 — Landforms and their Evolution
Alluvial Fans
Alluvial Fans
The photograph shows a broad, low cone of coarse debris spreading outward from the mouth of a steep hill stream. The stream emerges from a narrow mountain valley and suddenly meets a much gentler foot-slope plain, where its gradient drops sharply. In the foreground, the fan surface is littered with boulders, gravel, and other coarse material — the kind of heavy load a fast mountain stream can carry but cannot keep moving once the slope flattens. The fan widens as it extends away from the mountain front, giving it the classic cone or fan shape seen from above.
The key teaching point is the sudden loss of carrying power. On the steep mountain slope, the stream has high velocity and can transport very coarse sediment. The moment it breaks onto the low-gradient plain, that velocity drops, the load becomes too heavy, and the stream dumps its coarsest material first. Over time, repeated dumping builds up a cone-shaped deposit that spreads laterally — hence the name "alluvial fan." The coarsest debris settles nearest the apex (where the stream exits the hills), while finer material is carried a little farther out, though the photograph's main emphasis is on the overall coarse, spread-out nature of the deposit. …
Alluvial fans are landforms built where a stream suddenly loses its carrying power. The key to understanding them is the change in slope: a stream that has been rushing down a steep mountain slope, carrying a heavy load of coarse material, suddenly emerges onto a flat plain at the mountain's foot. That abrupt loss of gradient is the whole story.
When a stream flows over a mountain slope, its gradient is steep and its velocity is high. This allows it to transport a very coarse load — gravel, pebbles, boulders, and other large debris. But the moment it breaks out onto the foot slope plains, the gradient drops sharply. The stream can no longer maintain the energy needed to carry that heavy material.
The load becomes too heavy for the stream to transport over the gentler gradient. As a result, the stream dumps its sediment right there, at the base of the slope. This deposited material spreads out in a distinctive shape.
The accumulation of this dumped sediment forms a broad, cone-shaped landform. The book describes it as a "broad low to high cone shaped" feature — meaning the cone can range from relatively flat and low to more pronounced and elevated, depending on the volume of material deposited. This cone is the alluvial fan.
- Where they form: At the foot of mountain slopes, where streams transition from steep to gentle gradients.
- What builds them: Coarse load carried by streams flowing over mountain slopes.
- Why they form: The load becomes too heavy for the stream to carry once the gradient becomes gentler. …
Alluvial fans form when streams flowing from higher levels break into foot slope plains of low gradient. Normally very coarse load is carried by streams flowing over mountain slopes; this load becomes too heavy for the streams to carry over gentler gradients and gets dumped and spread as a broad low to high cone shaped deposit called an alluvial fan. The streams flowing over fans are not confined to their original channels for long and shift their position across the fan, forming many channels called distributaries. Alluvial fans in humid areas show low cones with ge …