Pre-dam

A forebay is an artificial pool of water in front of a larger body of water. The larger body of water may be natural or man-made.

Forebays have a number of functions. They are used in flood control to act as a buffer during flooding or storm surges, impounding water and releasing in a controlled way into the larger waterbody. They may be used upstream of reservoirs to trap sediment and debris (sometimes called a sediment forebay) in order to keep the reservoir clean. This may entail the use of a forebay dam or pre-dam. They may also be used upstream of lakes to prevent siltation. Some forebays are used simply to create a natural habitat for flora and fauna, to counterbalance the environmental impact of a dam or reservoir. Forebays vary greatly in size depending on their situation and purpose.

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Wikipedia