weir

A weir is a barrier placed in a river, stream, or other flowing water, and over which water is allowed to continue flowing. It may or may not create a larger body of water upstream, but unlike a dam its purpose is not to block the flow of water (using controlled flow to generate power) but rather to regulate or otherwise control the water's flow.

Water continues over the barrier rather than passing through movable gate openings. The upstream water surface can change without reservoir creation being its defining purpose.

A barrage provides controlled passages through gates. The weir instead regulates flow through its overflow form.