Which of the following best describes an alluvial fan's formation?

The formation of an alluvial fan is best described by the interaction between a mountain stream and the flatter land it encounters. When the fast-moving water from a stream flowing down a slope reaches a relatively flat area, it loses velocity, leading to a rapid deposition of sediment. This sediment builds up in a fan-shaped deposit, characteristic of alluvial fans.

The process typically occurs where the topography changes from steep to gentle, facilitating this sediment distribution. This makes the second choice the most accurate in capturing the essence of how alluvial fans are formed.

The option regarding sediment buildup during heavy rains, while somewhat related, does not fully capture the specific geomorphological process involved and can occur in various contexts. The one about seismic activity is unrelated to the formation process of alluvial fans, as seismic activity typically refers to ground motion and does not contribute directly to sediment deposition patterns in this context. Lastly, glacial movement contributes to different types of landforms such as moraines and does not pertain to the alluvial fan formation process.