A spillway is usually found by a dam. It is an area below a dam where extra water is allowed to drain away from the lake or reservoir above.

Spillway chute on the Itaipu Dam	Water discharged back into the Parana River from the spillway

The people who work at the dam and reservoir can calculate the maximum amount of water that should be in the reservoir. If there is too much water in the reservoir, the extra water can be drained off over the spillway. Without a spillway, the water may seep over the top of the dam, causing damage.

There are three types of spillways: overflow, chute and shaft. Chute spillways are often found by embankment dams. Shaft spillways are found on arch type dams. Overflows are found on all other types of dams. The overflow is the most common type of spillway.