Reservoir/ˈrezərvwɑːr/ Definition 1 of 3Pro Tip 1/3Physical Storage Refers to physical places where materials like water or oil are stored for use. The new dam will create a vast reservoir for irrigation purposes. Pro Tip 2/3Controlled Release A reservoir usually implies controlled release or usage of what it stores. They opened the reservoir gates to prevent flooding after heavy rainfall. Pro Tip 3/3Artificial vs Natural Can be man-made, like a water tank, or natural, like a lake used as a water source. The natural reservoir feeds the entire ecosystem with fresh water.