head gate vs floodgate

head gate and floodgate both are nouns.

Word	Noun	Adjective	Verb	Adverb
head gate	Yes	No	No	No
floodgate	Yes	No	No	No

As nouns, head gate and floodgate are synonyms defined as:

head gate and floodgate: regulator consisting of a valve or gate that controls the rate of water flow through a sluice

Other synonyms of head gate include penstock, sluice valve, sluicegate, water gate.

head gate (noun)	floodgate (noun)
a gate upstream from a lock or canal that is used to control the flow of water at the upper end	regulator consisting of a valve or gate that controls the rate of water flow through a sluice
regulator consisting of a valve or gate that controls the rate of water flow through a sluice	something that restrains a flood or outpouring