Anything that resembles a water slide with a gate is a sluice — a narrow channel that controls water flow.
The word originally comes from an old English word meaning a narrow channel that controlled a flow of water, usually to a watermill, and it still has that meaning. A sluice can also mean a splash — often with cold water and usually done with some vigor. A rude, watery slap in the face. Torture victims in movies inevitably have their faces sluiced to bring them round for further questioning.