As used in this article, unless the context requires a different meaning: “Alteration” means changes to an impounding structure that could alter or affect its structural integrity. Alterations include, but are not limited to, changing the height or otherwise enlarging the dam, increasing normal pool or principal spillway elevation or physical dimensions, changing the elevation or physical dimensions of the emergency spillway, conducting necessary repairs or structural maintenance, or removing the impounding structure. “Board” means the Soil and Water Conservation Board. “Construction” means the construction of a new impounding structure. “Dam break inundation zone” means the area downstream of a dam that would be inundated or otherwise directly affected by the failure of a dam. “Height” means the structural height of a dam which is defined as the vertical distance from the natural bed of the stream or watercourse measured at the downstream toe of the dam to the top of the dam. “Impounding structure” means a man-made structure, whether a dam across a watercourse or other structure outside a watercourse, used or to be used to retain or store waters or other materials. The term includes: (i) all dams that are twenty-five feet or greater in height and that create an impoundment capacity of fifteen acre-feet or greater, and (ii) all dams that are six feet or greater in height and that create an impoundment capacity of fifty acre-feet or greater. The term “impounding structure” shall not include: (a) dams licensed by the State Corporation Commission that are subject to a safety inspection program; (b) dams owned or licensed by the United States government; (c) dams operated primarily for agricultural purposes which are less than twenty-five feet in height or which create a maximum impoundment capacity smaller than 100 acre-feet; (d) water or silt retaining dams approved pursuant to § 45.2-618 or 45.2-1301; or (e) obstructions in a canal used to raise or lower water. “Owner” means the owner of the land on which a dam is situated, the holder of an easement permitting the construction of a dam and any person or entity agreeing to maintain a dam. “Watercourse” means a natural channel having a well-defined bed and banks and in which water normally flows.