4K Aerial footage with drone. A reservoir usually means an enlarged natural or artificial lake, storage pond or impoundment created using a dam or lock to store water. Reservoirs can be created by controlling a stream that drains an existing body of water. They can also be constructed in river valleys using a dam. A dam constructed in a valley relies on the natural topography to provide most of the basin of the reservoir. The valley sides act as natural walls. Water level decreased. Evaporation, vaporization, volatilization. Forest and highland.