Water level is the height of the surface of a body of water in relation to a set datum, usually sea level. Water levels also refer to the a point or boundary along a body of water that marks the average change from terrestrial to aquatic environments.
According to the Minnesota Department of Natural Resources, the ordinary high water line refers to the highest water line of water basins and other bodies of water. It also refers to the operating elevation of the average pool during summer.
Natural changes in weather and climate, such as tide, wind and disaster, make judging an official water level difficult to determine.