1) Watercourse 2) Waterway 
• (n.) See Race, a water course.
The specific mill-race for taking water to the waterwheel - whether that be an overshot (water hits the wheel from above) or undershot (water pushes the wheel from below) wheel.
Head'race` (-rās`) noun See Race , a water course. 
In a water wheel, a headrace is a channel above the wheel.
[n] - a waterway that feeds water to a mill or water wheel or turbine
noun a waterway that feeds water to a mill or water wheel or turbine