The addition to land by the slow action of water.
Heavy rain, river or ocean action would have this effect by either washing up sand or soil or by a permanent retreat of the high water mark.
The sudden washing up of soil is often called avulsion although the latter term is but a variety of accretion, just as alluvion refers to the resulting wash-up of soil or land: land that is deposited as a result of accretion.
In Parm, note this footnote to Justice King's reasons:
"Alluvion and accretion are used synonymously to describe the addition of soil by gradual deposit.
"[A]ny alluvion ... which forms along the banks of a river belongs to the riparian landowners who own the land adjacent to the river, when the river shifts course."
In US v Robertson Terminal, Justice Kennedy offered this full explanation of the law:
"Accretion refers to the increase of riparian land by the gradual deposit, by water, of solid material, whether mud, sand, or sediment, so as to cause that to become dry land which was before covered by water.
"Where a tract of land lies adjacent or contiguous to a navigable river, any increase of soil formed by the waters gradually or imperceptibly receding, or any gain by alluvion in the same manner... belongs to the proprietor of the adjacent or contiguous land.
"The land that is deposited as a result of accretion is known as alluvion, and riparian owners gain title to the deposited alluvion.
"This alluvion may result from natural forces or artificial forces, such as the construction of jetties upland.
"Accretion does not refer to the purposeful addition of land to waterfront property through laying fill and construction of wharves.
"Under the common law, a littoral owner cannot extend its own property into the water by landfilling or purposefully causing accretion.
"Because the fast lands at issue here consist of wharves and fill, the doctrine of accretion does not apply."
In Clarke, Canada's Supreme Court wrote:
"[A]ccretion denotes the increase which land bordering on a river or on the sea undergoes through the silting up of soil, sand or other substance, or the permanent retiral of the waters. This increase must be formed by a process so slow and gradual as to be, in a practical sense, imperceptible, by which is meant that the addition cannot be observed in its actual progress from moment to moment or from hour to hour, although, after a certain period, it can be observed that there has been a fresh addition to the shore line. The increase must also result from the action of the water in the ordinary course of the operations of nature and not from some unusual or unnatural action by which a considerable quantity of soil is suddenly swept from the land of one man and deposited on, or annexed to, the land of another.
"The fact that the increase is brought about in whole or in part by the water, as the result of the employment of artificial means, does not prevent it from being a true accretion, provided the artificial means are employed lawfully and not with the intention of producing an accretion, for the doctrine of accretion applies to the result and not to the manner of its production."