Final answer:
Owner A has likely gained additional land area through the process of accretion, which is a natural, gradual addition of land due to the receding high water mark of the adjacent river.
Step-by-step explanation:
The high water mark in the river next to Owner A's property has permanently receded. Owner A has likely gained land area by the process of accretion, which is the gradual and imperceptible deposit of earth material by natural causes, adding to the land adjacent to the water.
When a river or other body of water recedes, this can lead to an increase in the land area for the adjacent property holder, sometimes significantly altering property boundaries that were once defined by the natural high water mark. The concept of accretion is one aspect of property law that deals with changes in land ownership as a result of natural alterations in land boundaries.
This process can be contrasted against avulsion, which refers to the sudden and perceptible loss or addition of land by actions such as a flood or landslide, and does not immediately impact property boundaries. Understanding these processes is essential for matters involving real estate, land use, and property law. Moreover, discussion on compensation may arise if the addition of land substantially alters the value of the property or affects the public use of the waterway.