A Spillway is a structure used to regulate the flow of water in conjunction with a dam, levee, or other water contol device. Specifically, it is a structure which conveys water over or through a dam when a reservoir is full.

There are two types of spillways. Controlled and uncontrolled.

A controlled spillway has a mechanical method to regulate the water flow over or through a dam. This can be a door or series of door, valves, or other methods of diverting or preventing the overflow of water.

Uncontrolled spillways are only regulated by the spillway's physical dimensions. For example; if the spillway is 18 feet high, it will only control 18 feet of water. Any excess will go over the dam.