WEIR

(1) A barrier placed in a channel to divert fish or water. 

(2) A dam in a river to stop and raise the water, for the purpose of conducting it to a mill, forming a fishpond, or the like.

Automated working small weir in a stream manages the water level in a Dutch polder. The flowing water foams. The weir reflects in the mirror-smooth water surface of the ditch.