What is Alluvion?

Legal Definition
Alluvion is a legal term describing the increase in the area of land due to sediment (alluvium) deposited by a river. This changes the size of a piece of land (a process called accession) and thus its value over time.
-- Wikipedia
Legal Definition
A gradual increase or decrease in land that will change the property lines (for example, as when a stream that serves as a property line slowly changes course). Compare to avulsion (a sudden change which will not alter the property boundry). See accretion.
Legal Definition
The insensible increase of the earth on a shore or bank of a river by the force of the water, as by a current or by waves. It is a part of the definition that the addition, should be so gradual that no one can judge how much is added at each moment of time. Just. Inst. lib. 2, tit. 1, 20; 3 Barn. & Cress. 91; Code Civil Annote No. 556. The proprietor of the bank increased by alluvion is entitled to the addition. Alluvion differs from avulsion in this: that the latter is sudden and perceptible. See avulsion. See 3 Mass. 352; Coop. Justin. 458; Lord Raym. 77; 2 Bl. Com. 262, and note by Chitty; 1 Swift's Dig. 111; Coop. Just. lib. 2, t. 1; Angell on Water Courses, 219; 3 Mass. R. 352; 1 Gill & Johns. R. 249; Schultes on Aq. Rights, 116; 2 Amer. Law Journ. 282, 293; Angell on Tide Waters, 213; Inst. 2, 1, 20; Dig. 41, 1, 7; Dig. 39, 2, 9; Dig. 6, 1, 23; Dig. 1, 41, 1, 5; 1 Bouv. Inst. pars 1, c. 1 art. 1, 4, s. 4, p. 74.
-- Bouviers Law Dictionary
Legal Definition
That increase of the earth on a shore or bank of a river or to the shore of the sea, by the force of the water, as by a current or by waves, which is so gradual that no one can judge how much is added at each moment of time. Inst. 1, 2, t. 1, ยง 20. Ang. Water Courses, 53. Jefferis v. East Omaha Land Co., 134 U. S. 178, 10 Sup. Ct 518, 33 L. Ed. 872; Freeland v. Pennsylvania EU Co., 197 Pa. 529, 47 Atl. 745, 58 L. ItA. 206, 80 Am. St. Rep. 850.

The term is chiefly used to signify a gradual increase of the shore of a running stream, produced by deposits from the waters. By the common law, alluvion is the addition made to land by the washing of the sea, or a navigable river or other stream, whenever the increase is so gradual that it cannot be perceived in any one moment of time. Lovingston v. St Clair Co.unty, 64 111. 58, 16 Am. Rep. 516.

Alluvion differs from avulsion in this: that the latter is sudden and perceptible. St. Clair County v. Lovingston, 23 Wall. 46, 23 L. Ed. 59. See Avulsion.
-- Black's Law Dictionary
Legal Definition
Gradual washing up of sand and earth so as to increase the quantity of land owned by a riparian proprietor. See 22 Am. St. Rep. 195.
-- Ballentine's Law Dictionary