A fast-running water-filled channel diverted from a river or stream used to drive a mill wheel.
1935, Benedict Read, The Green Child, Capuchin 2010, p. 36:
She liked the cold water of the mill-race, and without shame or hesitation would throw off her frock and float like a mermaid, almost invisible, in the watery element.
2000, Lewis J Swindle, The History of the Gold Discoveries of the Northern Mines of Californias Mother Lode Gold Belt As Told by the Newspapers and Miners 1848-1875'', Trafford Publishing, ISBN 155212472X, p.8:
The mill race was a ditch that was cut which started along the river’s edge that ran past the mill to provide the water that would turn a wheel that would provide power to the saws. There was a gate on the ditch which could be raised or lowered to allow the water to run or to stop its flow down the ditch.