According to the Oxford English Dictionary, "leat"' is cognate with "let" in the sense of "allow to pass through". Other names for the same thing include fleam (probably a leat supplying water to a mill that did not have a millpool). In parts of northern England, for example around Sheffield, the equivalent word is goit. In southern England, a leat used to supply water for water-meadow irrigation is often called a carrier, top carrier, or main.
Leats generally start some distance (a few hundred yards, or perhaps several miles) above the mill or other destination, where an offtake or sluice gate diverts a proportion of the water from a river or stream. A weir often serves to provide a reservoir of water adequate for diversion. The leat then runs along the edge or side of the valley, at a shallower slope than the main stream. The gradient determines the flow rate together with the quality of the wetted surface of the leat. The flow rate may be calculated using the Manning formula. By the time it arrives at the water mill the difference in levels between the leat and the main stream is great enough to provide a useful head of water – several metres (perhaps 5 to 15 feet) for a watermill, or a metre or less (perhaps one to four feet) for a water-meadow.
In mining areas, leats were built to supply water for hushing mineral deposits, washing ore and powering mills. They were used extensively by the Romans, and can still be seen at many sites, such as the Dolaucothi gold mines. They used the aqueducts to prospect for ores by sluicing away the over burden of soil to reveal the bedrock in a method known as hushing. They could then attack the ore veins by fire-setting, quench with water from a tank above the workings, and remove the debris with waves of water, a method still used in hydraulic mining. The water supply could then be used for washing the ore after crushing using simple machines also driven by water.