What do we call the greatest thickness of material deposited by a river?

The term describing the greatest thickness of material deposited by a river is alluvium. Alluvium refers specifically to the sediment that is deposited by flowing water in riverbeds, floodplains, and deltas. This sediment can include a variety of particle sizes, including sand, silt, clay, and organic material. When rivers flood or slow down, they lose energy, causing them to deposit their load, which contributes to the buildup of alluvium.

The other terms indicate specific types of sediment rather than the collective term for all river-deposited materials. Clay, silt, and sand are components of alluvial deposits but do not encompass the full range of materials that can accumulate in significant thicknesses along riverbanks and beds. Therefore, alluvium is the most appropriate term for the general category of material whose thickness can be substantial due to deposition processes associated with river dynamics.