An alluvial fan is a naturally occurring phenomenon that creates a cone-shaped deposit of sediment, crossed and made up of streams. They are usually found where a canyon draining mountainous terrain emerges onto flatter grounds. There are many such examples of these geographic features around the world and they have even been observed on Mars, supporting the theory that there must have once been water on the planet.

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Not just creating an interesting shape and a geometric natural wonder, alluvial fans also serve as diverse habitats for local flora and fauna. Plants with long roots can typically be found at the edge of the fan’s cone, where they will reach down into impermeable water basins for sustenance. These plants then provide food and shelter to many species of animals from the surrounding areas. While they make good homes in the natural world, they are typically places to avoid when it comes to human habitation, as the alluvial fans are prone to flooding in both arid and wet climates.

Some examples of the phenomenon can be caused without the aid of water, although technically these are called colluvial fans, and these are often created by mass erosion or landslides; essentially, they are built by a continuous downward movement of rocks and soil. The debris will collect in the familiar cone shape as it reaches flatter ground.

One of the largest and most prominent examples of an alluvial fan is the megafan at the Koshi River in Nepal. Aided by the wet climate in the region, the fan spreads over an area of around 15,000 square kilometers. Exiting the Himalayan foothills onto the level plains, the river itself traverses into India and eventually feeds into the Ganges. The risk of flooding is increased the larger the fan, however, and in August 2008 high monsoon flows breached the fans embankment. The overflow spilled into areas with a high population density and thousands of people lost their lives, homes, and livelihoods through crop destruction.

In the United States of America several smaller yet still impressively extensive alluvial fans are formed flowing into California’s Central Valley. The Kings River is perhaps the most notable and as it flows out of the Sierra Nevada it creates a low divide, which turns the southern end of San Joaquin Valley into an endorheic basin with no connection to the sea.

When more than one alluvial fan converge, it is known as a bajada. They are most common in drier climates such as canyons of the American Southwest. They can be either narrow, formed of two or three small streams, or fed by dozens, which creates a much wider convergence.

Alluvial fans can even be found underwater and are then known as subaqueous fans; these are created by underwater currents that deposit alluvium to form a submarine hill or glacier. These spectacular formations are of particular interest to divers.

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