Leat

The most famous examples are Drake's Leat[?], constructed on the instructions of Sir Francis Drake to conduct water from Dartmoor to Plymouth, and the Devonport Leat constructed in the late 18th century to carry water to the explanding naval dockyard at Devonport, now a part of Plymouth.

According to the Oxford English Dictionary, the word is cognate with let in the sense of "allow to pass" (as in "passing water").