A hydraulic structure that blocks the flow of water to prevent fish from escaping. Built at the top of a pond or on rivers and streams that flow into it.

A fish pond headwater is a wooden, concrete or reinforced concrete water structure that holds fish. It is placed at the top of the pond, above the source of water supply (river, stream, canal). The water entering the pond passes through grids that prevent large predatory and weedy fish from entering the farm and fish raised in the pond from leaving. The openings in the grids are 1-1.5 cm. A disadvantage of the headwall is the need to remove the grids when large floods pass through and debris clogs the grids. During this time, predatory and waste fish can enter the pond. It is therefore common to install 2 rows of grids which are removed one at a time to remove debris and fouling.