A spillway is a structure used to provide the controlled release of water downstream from a dam or levee, typically into the riverbed of the dammed river itself. Spillways ensure that water does not damage parts of the structure not designed to convey water. They can include floodgates and fuse plugs to regulate water flow and reservoir level, and such features enable a spillway to regulate downstream flow by releasing water in a controlled manner before the reservoir is full, preventing an unacceptably large release later.

Spillways can be classified into two types: controlled and uncontrolled. A controlled spillway has mechanical structures or gates to regulate the rate of flow. This design allows nearly the full height of the dam to be used for water storage year-round, and flood waters can be released as required by opening one or more gates. On the other hand, an uncontrolled spillway is set and forget, and water flows over it without any mechanical structures or gates to regulate the rate of flow.

Spillways are particularly important safety features for several reasons. First, the uncontrolled discharge of surplus water past the dam should be automatic and not dependent upon human control. Second, the spillway intake should be wide enough so that the largest floods can pass without increasing the water level in the reservoir enough to cause a nuisance to upstream property owners. Third, the rate of floodwater discharge should not increase much above that experienced before the construction of the dam.

Water normally flows over a spillway only during flood periods, when the reservoir has reached its capacity and water continues entering faster than it can be released. In contrast, an intake tower is a structure used to control water release on a routine basis for purposes such as water supply and hydroelectricity generation.

Designing a spillway usually requires a team of engineers performing structural, geotechnical, electrical, mechanical, hydrologic, and other analyses.