Alluvion is a legal term that describes an area of land that comes to exist due to the gradual increase of sediment deposited on the shoreline by a body of water through natural causes. Under real property law, the law by which a person establishes rights in and holds title to a parcel of land, property that borders a body of water, has its own rules regarding title that factor in the effects of nature on the border that serves as a shoreline. Establishing proper title to land as between competing parties has a history that goes all the way back to Roman law.
Alluvion is often used in legal matters in conjunction with accretion, the legal term for additional land gained through the operation of natural forces. Many people use the terms interchangeably, but this is incorrect. Accretion refers to the act of creating the addition, and alluvion refers to the physical sediment deposits.
An prime example of a grant of title in land within the legal definition of the development of alluvion is a stream that runs between two properties. When the owners of the property first make their purchase of land, the boundaries of the title are drawn based on the placement of the stream and seemed fixed. Over time, however, the stream may gradually change its course, causing the bank of one property to expand and the other contract. The boundary lines of the properties change with the course of the stream, and one owner loses what the other owner gains.
Any land gained through accretion automatically becomes the property of the owner of the existing land to which the alluvion attaches. The additional property must satisfy certain tests to be considered alluvion. It must have been formed through natural causes, specifically through gradual and imperceptible amounts that it is not possible to judge exactly how much land was added at any given time. Sudden and perceptible additions of land is called avulsion. Unlike alluvion, land created by avulsion remains property of owner from whom the land was taken by forces of nature.
The perceptible differences between accretion and avulsion demonstrates why land titles transfer with accretion and do not with avulsion. With accretion, the redistribution is so gradual that the original owner can no longer identify his property. Gradual accumulation is the key element to title transfer, not just the fact that new land has been gained through accretion.