In real property law, avulsion refers to a sudden loss or addition to land, which results from the action of water. It differs from accretion, which describes a gradual loss or addition to land resulting from the action of water.
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Avulsion and riparian owners
The distinction between avulsion and accretion becomes important where a river forms the boundary between two riparian owners. In many states (though not all), if the river changes channels by avulsion, the boundary does not change; it remains in the middle of the old channel. (This is why if you look at maps of the lower Mississippi river, you will find that some land on the east side of the river is part of Arkansas, and some land on the west side of the river is part of Tennessee. The river changed course quickly, so the state boundary did not change.) However, as a river gradually changes through accretion, the boundary changes with it.
To prove that a change was avulsion and not accretion, it is sufficient for the owner of land which was washed away to point out approximately as much land added to the opposite bank as washed away from his bank.
Avulsion and littoral owners
Avulsion can also affect littoral owners through events like hurricanes that can rapidly erode a shoreline. Courts have determined that littoral owners have the right to all the land that they had previous to the avulsive event.