Editing Leat

{{Short description|Artificial watercourse or aqueduct dug into the ground}}
{{Use dmy dates|date=March 2025}}
{{Use British English|date=December 2016}}
[[File:Devonport leat 6.jpg|thumb|200px|The Devonport leat near Nun's cross farm]]

A '''leat''' ({{IPAc-en|'|l|i:|t}}; also '''lete''' or '''leet''', or '''millstream''') is the name, common in the south and west of England and in Wales, for an artificial [[watercourse]] or [[Aqueduct (watercourse)|aqueduct]] dug into the ground, especially one supplying water to a [[watermill]] or its [[mill pond]]. Other common uses for leats include delivery of water for [[hydraulic mining]] and [[Buddle pit|mineral concentration]], for [[irrigation]], to serve a [[Dyeing|dye]] works or other industrial plant, and provision of drinking water to a farm or household or as a catchment cut-off to improve the yield of a [[reservoir]].

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''.

==Design and functions==

===Water mills===
Leats generally start some distance (a few hundred metres/yards, or perhaps several miles/kilometres) 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]] in the source stream 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, together with the quality of the wetted surface of the leat, determines the [[Volumetric flow rate|flow rate]]. 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 [[hydraulic head|head]] of water &ndash; several metres (perhaps 5 to 15 feet) for a watermill, or a metre or less (perhaps one to four feet) for the controlled irrigation of a [[water-meadow]].{{Citation needed|date=May 2024}}

===Water supply===
Leats are used to increase the yield of a reservoir by trapping streams in nearby catchments by means of a contour leat. This captures part or all of the stream flow and transports it along the contour to the reservoir. Such leats are common around reservoirs in the uplands of Wales.{{Citation needed|date=May 2024}}

===Mining===
[[File:Dolaucothimap2.jpg|thumb|upright=1.2|Map of the Roman gold mine]]
[[File:Aqueduct1.jpg|thumb|left|upright=1.2|The aqueducts at Dolaucothi]]
Leats were built to work [[lead]], [[tin]] and [[silver]] ores in mining areas of Wales, [[Cornwall]], [[Devon]], the [[Pennines]] and the [[Leadhills]]/[[Wanlockhead]] area of Southern Scotland from the 17th century onwards. They were used to supply water for [[hushing]] mineral deposits, washing ore and powering mills.{{Citation needed|date=May 2024}}

==Use in Roman times==
Leats were also used extensively by the [[Ancient Rome|Romans]], and can still be seen at many sites, such as the [[Dolaucothi Gold Mines|Dolaucothi]] goldmines. They used the [[Aqueduct (watercourse)|aqueducts]] to [[Prospecting|prospect]] for ores by sluicing away the overburden 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 by simple machines also driven by water.{{Citation needed|date=May 2024}}

The Romans also used them for supplying water to the bath-houses or [[thermae]] and to drive vertical [[water wheels|water-wheels]].{{Citation needed|date=May 2024}}

[[File:Devonport leat 3.jpg|thumb|Devonport leat showing sluice gates]]

==Dartmoor==
There are many leats on [[Dartmoor]],<ref>[http://www.legendarydartmoor.co.uk/leats_moor.htm Tim Sandles, 21 March 2016: Leats] at legendarydartmoor.co.uk, accessed 5 April 2018</ref> mostly constructed to provide power for mining activities, although some were also sources of drinking water.  The courses of many Dartmoor leats may still be followed.<ref>Robins, J. (1984) ''Follow the leat with John Robins: a series of walks along Dartmoor leats and a description of the mines some of them served'', John Robins, {{ISBN|0-9508030-0-6}}</ref><ref>Hawkings, D.J. (1987) ''Water from the moor'', Devon, {{ISBN|0-86114-788-X}}  Provides a full history of the leats which supplied Plymouth, England.</ref> Many such leats on the moor are marked on the 1:50000 and 1:25000 [[Ordnance Survey]] maps, such as that serving the now-defunct [[Vitifer mine]] near the [[Warren House Inn]]. Notable leats include:
*[[Drake's Leat]], constructed in 1591 under the management of Sir [[Francis Drake]], as an agent of the Corporation of Plymouth, to carry water from [[Dartmoor]] to [[Plymouth]].
*[[Devonport Leat]] constructed in the late 18th century to carry water to the expanding naval dockyard at [[Devonport, Devon|Devonport]] (now a part of Plymouth).

==See also==
* [[Acequia]]
* [[Aqueduct (watercourse)]]
* [[Flume]]
* ''[[Kunstgraben]]''
* [[Mill race]]
* [[Penstock]]
* [[Roman aqueduct]]
* [[Roman engineering]]
* [[Roman mining]]

==References==
{{Reflist}}

==External links==
{{wiktionary|leat|millstream}}

{{Rivers, streams and springs}}

[[Category:Aqueducts in the United Kingdom]]
[[Category:History of mining]]
[[Category:Hydraulic engineering]]
[[Category:Water management in mining]]

[[cs:Náhon]]
[[nrm:Bieu]]