A sluice gate is traditionally a wood or metal barrier sliding in grooves that are set in the sides of the waterway. Sluice gates commonly control water levels and flow rates in rivers and canals. They are also used in wastewater treatment plants and to recover minerals in mining operations, and in watermills.
"Sluice gate" refers to a movable gate allowing water to flow under it. When a sluice is lowered, water may spill over the top, in which case the gate operates as a weir. Usually, a mechanism drives the sluice up or down. This may be a simple, hand-operated, chain pulled/lowered, worm drive or rack-and-pinion drive, or it may be electrically or hydraulically powered.
The design of a Van gate is shown in the image on the lower right. The sluice has a separate chamber that can be filled with water and is separated by the high water level side of the sluice by a large door. When a tube connecting the separate chamber with the he high water level side of the sluice is opened, the water level and with that the water pressure in this chamber will raise to the same level as that on the high water level side. The surface of the door separating the chamber from the high water level side of the sluice is larger than that of the door closing the sluice. Since pressures are equal this results into a net force that opens up the sluice.
In the mountains of the United States, sluices transported logs from steep hillsides to downslope sawmill ponds or yarding areas. Nineteenth-century logging was traditionally a winter activity for men who spent summers working on farms. Where there were freezing nights, water might be applied to logging sluices every night so a fresh coating of slippery ice would reduce friction of logs placed in the sluice the following morning.
Sluice boxes are often used in the recovery of black sands, gold, and other minerals from placer deposits during placer mining operations. They may be small-scale, as used in prospecting, or much larger, as in commercial operations, where the material is first screened using a trommel or screening plant. Typical sluices have transverse riffles over a carpet, which trap the heavy minerals, gemstones, and other valuable minerals. The result is a concentrate.
In the Somerset Levels, sluice gates are known as clyse or clyce.