Alluvial and fluvial are similar terms in that they both refer to deposits created by moving water, usually in the form of rivers. They differ in that some alluvial deposits, such as alluvial fans, can be formed by processes besides rivers, such as flash floods, whereas fluvial deposits are always deposited by rivers.

Fluvial pertains to rivers and the processes associated with them. It involves everything relating to the dynamics and movement within river systems. Alluvial, on the other hand, is focused on the sediments or deposits left behind by these flowing waters.

Where the river flow is fast, more particles are picked up than dropped. Where the river flow is slow, more particles are dropped than picked up. Areas where more particles are dropped are called alluvial or flood plains, and the dropped particles are called alluvium.