What creates an alluvial fan?

An alluvial fan is created when sediment is deposited by fast-moving mountain streams at the point where they emerge from steep mountainous terrain and enter a flatter landscape. This process occurs because as the stream floods out of the mountains, its velocity decreases suddenly upon entering a wider, less steep area. Consequently, the sediment carried by the water settles out of suspension and accumulates in the fan-like shape characteristic of alluvial fans.

The formation of an alluvial fan is influenced by factors such as the gradient of the slope, the volume of sediment being transported, and the energy of the water flow. Over time, this deposition can lead to the creation of a distinct landform that diverges outward in a triangular shape from the mountain front.

Other options do not accurately describe the process of alluvial fan formation. For instance, the expansion and contraction of lakes relate to different sedimentary processes and do not create alluvial fans. Rivers meandering through flat landscapes create a different landform known as a floodplain, and glacial meltwater flowing might contribute to sedimentary processes but does not form an alluvial fan as described in the context of mountain streams.