A sluice is a water channel that is controlled at its head by a gate. For example, a millrace is a sluice that channels water toward a water mill.
A sluice gate is traditionally a wooden or metal plate which slides in grooves in the sides of the channel. Sluice gates are commonly used to control water levels and flow rates in rivers and canals. They are also used in wastewater treatment plants and to recover minerals in mining.
We have lots of canals thus lots of bridges.