A spillway is located at the top of the reservoir pool. Dams may also have bottom outlets with valves or gates which may be operated to release flood flow, and a few dams lack overflow spillways and rely entirely on bottom outlets.
There are two types of spillways: 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.
An uncontrolled spillway, in contrast, does not have gates; when the water rises above the lip or crest of the spillway it begins to be released from the reservoir. The rate of discharge is controlled only by the depth of water within the reservoir. All of the storage volume in the reservoir above the spillway crest can be used only for the temporary storage of floodwater, and cannot be used as water supply storage because it is normally empty.
In an intermediate type, normal level regulation of the reservoir is controlled by the mechanical gates, but if inflow to the reservoir exceeds the gates capacity, an artificial channel blocked by a fuse plug dike will operate. The fuse plug is designed to overtop and wash out in case of a large flood, greater than the discharge capacity of the spillway gates. Although it may take many months to restore the fuse plug and channel after such an operation, the total damage and cost to repair is less than if the main water-retaining structures had been overtopped. The fuse plug concept is used where it would be very costly to build a spillway with capacity for the probable maximum flood.
Some spillways are designed like an inverted bell so that water can enter all around the perimeter. These uncontrolled spillway devices are also called morning glory, plughole, glory hole, or bell-mouth spillways. In areas where the surface of the reservoir may freeze, bell-mouth spillways are normally fitted with ice-breaking arrangements to prevent the spillway from becoming ice-bound. Chaffey Dam, located near Tamworth, New South Wales in Australia has a classic example of an inverted-bell spillway. It was the first one created in Australia of its kind.