What concept refers to property lines gradually shifting due to natural causes?

The concept that refers to property lines gradually shifting due to natural causes is known as accretion. This process occurs when land is added to a property as a result of the deposition of soil by the action of water, such as in riverbanks or coastlines. Over time, this can lead to an increase in the size of the property as new land forms.

Accretion is significant in real estate because it can influence ownership and property rights. As land gradually accumulates, the owner of the adjacent property may gain additional land, which can affect boundaries and may require legal documentation to establish the new property lines.

The other concepts listed are important to understand in the context of land and property as well, but they do not define the gradual increase of property due to natural processes which is specifically the essence of accretion. Alluvion refers specifically to the soil or material deposited by water, while erosion is the process of land being worn away, and encroachment typically involves the unlawful intrusion of a structure or property onto another's land.