Here's why:
* Alluvium is a general term for unconsolidated (loose) sediment deposited by a stream. This sediment can range in size from clay to boulders.
* Fluvial is a term related to rivers and streams, so you might also hear it used in the context of stream deposits.
It's important to note that the term "alluvium" doesn't specify the specific type of sediment, just its origin. The composition of alluvium can vary depending on the source rocks, the stream's flow, and other factors.
