The Selinda spillway fills with floodwaters from the Angolan hills once every 20 or 30 years, bringing tilapia, hippos, pelicans, snub-nosed ducks and rafts of water lily which locals eat as a delicacy.
Vertical lift, or radial, gates rise to permit flow under the gate but over the spillway crest.
Drum gates present the fewest problems in the passage of ice or drift over the spillway.
The spillway consists of a concrete dike along the east (left) bank of the Mississippi and the spillway itself, a broad lowland that extends northeastward for about 6 miles (10 km) to the lake and is contained by levees.
The construction of a spillway in the area was proposed in 1927 after a major flood that year killed more than 500 people.
With embankment dams, a separate spillway structure is normally constructed to one side of the dam.
In some dams, the powerhouse is constructed on one flank of the dam, part of the dam being used as a spillway over which excess water is discharged in times of flood.
A facing of crushed stone prevents erosion by wind or rain, and an ample spillway, usually of concrete, protects against catastrophic washout should the water overtop the dam.
A spillway step constructed at Włoclawek in 1968 initiated a series of improvements that continued through the 1980s.
The excavator has inferred that the basin was a dock to which ships could be brought from the nearby estuary via an artificial channel that would have been kept clear of silt by controlling the flow of water from the spillway.
When fully opened, the spillway can divert some 250,000 cubic feet (7,100 cubic metres) of water per second.
