term:

Alluvion

alluvion n

: material (as clay, silt, sand, or gravel) deposited by running water
;esp
: the land added by the gradual or imperceptible accumulation of such material along a bank or shore [ formed in front of the property of several owners is divided equitably "Louisiana Civil Code"]

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.