Head gate and Weir

Head gate noun - A gate upstream from a lock or canal that is used to control the flow of water at the upper end.

Weir and head gate are semantically related. In some cases you can use "Weir" instead a noun phrase "Head gate".

Weir noun - A low dam built across a stream to raise its level or divert its flow.

Head gate and weir are semantically related. You can use "Head gate" instead a noun "Weir".