Click the photo to see the full-size version of Deception Canyon fan, near Palm Springs. When mountains shed sediment off their flanks, streams carry it away as alluvium
. A mountain stream carries lots of alluvial sediment easily when its gradient is steep and energy is abundant. When the stream leaves the mountains and debouches onto the plain, it drops most of that alluvial sediment immediately. So over thousands of years, a wide cone-shaped pile builds upan alluvial fan. A steep-sided fan may instead be called an alluvial cone.
Alluvial fans are also known on Mars.