Which of the following best describes alluvium?

Alluvium refers to material, typically sediment, that has been deposited by the action of flowing water, such as rivers or streams. This sediment is generally characterized by having been sorted by the natural processes of transportation and deposition. The process of alluvial deposition involves the sorting of particles according to size and density, which occurs as water slows down and loses energy, allowing heavier particles to settle first.

In contrast, unsorted soil, organic material, and hard rock do not fit the definition of alluvium. Unsorted soil might contain a mix of particle sizes that haven’t undergone sorting, organic material refers to decomposed plant and animal matter rather than inorganic sediment, and hard rock typically refers to consolidated material that has not been shaped by sedimentation processes. Thus, sorted sediment is the most accurate description of alluvium, making it the correct answer.