The term you're looking for is "alluvial". Alluvial refers to something related to or consisting of fertile soil that has been deposited by a river. It is often used to describe land or soil that is located near rivers or riverbeds, where natural processes like flooding and sediment deposition have resulted in the accumulation of rich and fertile soils. The word "alluvial" may also be used to describe geological formations or landscapes created by the deposition of river-borne sediments.