Where mountain streams hit flat ground, they spread sediment into giant fans
When a stream bursts from a narrow canyon onto open ground, it suddenly slows and dumps its load, building a cone of gravel and sand. These alluvial fans range from a few metres wide to nearly 20,000 square kilometres, and they turn up on Mars and Titan as well as Earth.
The secret is the sudden loss of carrying power. Confined in a channel, water or a slurry of debris can move heavy loads; spread thin across a plain, or soaking into it, it cannot. A river that leaves the mountains without slowing tends to build a broad alluvial plain instead. Because the physics is just gravity and geometry, similar fans on Mars and Saturn's moon Titan point to past flowing liquid there, and fans on Mars with eroded edges mark former river systems.
A typical fan is shaped like a slice of a shallow cone, sloping between 1.5 and 25 degrees. It is steepest and coarsest near the apex and flattens toward the rim, with poorly sorted sediment throughout. Near the top, flow usually runs in a single trench that can be 30 metres deep. When debris clogs it, the water breaks out and switches to a steeper part of the fan, a jump called nodal avulsion, so only a slice of the fan is active at once while abandoned areas weather into soil.
Such switches can be deadly. The biggest fans line the foot of the Himalaya on the Indo-Gangetic Plain, and in 2008 a shift in the feeder channel of the Kosi River fan caused catastrophic flooding.
Fans form mostly at the base of desert ranges, as in North America's Great Basin, south Devon's New Red Sandstone and the populous oases of Xinjiang around the Taklamakan Desert and Junggar Basin. Where many streams leave a mountain front, their fans merge into a continuous apron called a bajada. Some fans are built by streams, others by debris flows: slurries of clay-to-boulder material resembling wet concrete that stiffen as they slow and can stop on a slope. Those are commonest where mudstone or clay-rich rock supplies fine sediment and intense thunderstorms saturate hillsides until they fail. Waves or rivers nibbling a fan's edge leave a small cliff, and such toe-trimmed fans can record climate or tectonic change and may improve a fan's value as an aquifer or oil reservoir.
Source: Alluvial fan
