A channel or canal that carries water from a source down to a water wheel or turbine in a mill.
From 'head' (upper part, source) and 'race' (a channel for water flow, from Old Norse 'rás'). The compound emerged in Middle English as mills became common features of medieval landscapes.
Medieval millers were basically hydraulic engineers—they designed these 'headraces' to create the perfect slope and water pressure, and these channels were so important that people fought legal battles over water rights.
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