Did You Know?

The largest alluvial fan in the world is present in the Kunlun and Altun mountain ranges of the Taklimakan desert in XinJiang, China. It is 35.16 miles wide and 38.09 miles long. Parts of the fan are blue, and alive with the water flowing from the Molcha river.

In Earth science, by definition, an alluvial fan is a fan-shaped deposit that gets formed when a stream of water flowing rapidly down a steep gradient enters an area having a gentle slope, slows down, flattens, and spreads out depositing gravel, sand, and smaller pieces of sediment, such as silt―together known as alluvium―in that area. Alluvial fans are typically found in mountainous locations, where water draining through a canyon exits out onto a flat plain, especially near fault-bounded mountain fronts. They may even be formed out of debris flow, in which case they are known as debris cones or colluvial fans.

Alluvium fans are formed as a result of the interaction of flowing water with mountains, hills, or the walls of a canyon. This water may comprise trickles of rainwater, a rapidly flowing creek, a strong river-flow, or even runoffs from industries. The steep slope of the mountainous terrain causes the water to pick up speed and flow aggressively. Its strong current carves the surrounding landscape, breaking apart and carrying small chunks of rocks, gravel, and sand with it. As this rushing water enters the plains, the gentler slope causes it to slow down. Due to its decreased velocity, the water no longer possesses enough energy to carry the coarse-grained alluvium along, and therefore, this gets deposited in the plain area. As more and more alluvium continues to be deposited, the accumulation becomes big enough to block the flow of water, causing the original channel of flow to split as water tries to flows around it. This process continues till, over a period of time, many distributed channels of flowing water create a widely-spread triangular fan-shaped feature, known as an alluvium fan. The narrow portion of the alluvium fan is known as its apex, and its widely-spread triangular region is known as the fan's apron. Alluvium fans can be tiny, having an apron-spread only a few centimeters, or they can be enormous, with their aprons spanning thousands of square miles. For example, the Koshi river in Nepal has, over a period of time, built up an alluvial fan that is more than 5800 mi² (15000 km²) wide. It is aptly called a mega-fan, and has been formed due to the alluvium carried down by the river from the Himalayan mountains.

Bajada When two or more alluvial fans converge or overlap, they form what is known as a Bajada. Bajadas can be narrow, made from the flow of only a couple of streams, or can be large and wide, resulting from the convergence of several different alluvial fans. Bajadas are frequently found in arid climates, such as for example, in the canyons of southwest America. They are also found in wetter climates where several streams flow. Debris Cone An alluvial fan with a steep slope, having a shape closer to a half fan, is known as a debris cone. These form due to the slow accumulation of alluvium over many hundreds of years. They also arise as large boulders and other material accumulate during landslides, floods, etc. Subaqueous Alluvial Fans Submarine hills and valleys found at the floors of large water bodies also give rise to alluvial fans. They are typically formed because of the alluvium deposited due to a strong water current. Colluvial Fans Alluvial fans can even form in the absence of water. They are called colluvial fans, and are typically a consequence of mass-wasting caused by the downward movement of rocks, soil, and other materials. An example of such mass-wasting is a landslide.

In alluvial fans, tress such as ash or willow and shrubs, including rabbitbrush and greasewood, are commonly found. These plants comprises very deep roots, which help them access the water that has sunk in deep below the alluvium cover of the fan. They also help anchor them firmly on the rather loose surface. Settling on or close to an alluvium fan can be dangerous, as they are subject to flooding. Rushing water, debris, mud, and even boulders may pose a threat, even miles away from the apex of the fan.

Alluvial fans have a steep slope, coarse-grained sediments which may include boulders, and are usually caused by large flash-floods or debris flows. River deltas, on the other hand, have a shallow slope, comprise fine-grained sediment including sand and mud, and are usually formed by the spreading of a river where it enters a body of water, such as seas, oceans, etc.

Alluvial fans aren't just restricted to our planet Earth. They have been found to exist on Mars, descending from some steep crater rims to their flatter floors. These formations have been observed by orbiting satellites in the Gale Crater. They have been confirmed by the discovery of fluvial sediments by NASA's Mars rover - Curiosity. Alluvial fans have also been located by the Cassini-Huygens mission on Titan, which is the largest of the moons orbiting the planet Saturn. These observations were made via the Cassini Orbiter's Synthetic Aperture Radar (SAR) instrument, and are commonly found in the drier middle latitudes at the ends of the methane/ethane rivers, where frequent wetting and drying occurs due to precipitation, similar to the arid fans on Earth. Analysis of the images suggest that the fan material is composed of round grains of water ice, and solid organic compounds that are about two centimeters in diameter.