1) Literally the front embankment: the word ‘bay’ could mean a dam or embankment and by an extension of its meaning ‘forebay’ came also to allude to the reservoir.
The few references located so far all have to do with water mills and their dams: 1612-6 all the tymber that shalbe needful for ground works … also sufficient soyle trees … and plankes for the whole forebaye, Golcar
1699 all the grounds, shuttles, forebays, Kirkheaton. Timber was evidently used in the construction of the dam and further details emerge in 1712-3, in the cash accounts for Rivelin Mill in Sheffield: they include payment to a workman for 4 days loading Clay to be Rammed in the forebay there. An arbitration award in 1795 required one miller to make a covered goit thro’ John Armytage’s lands to the ancient goit Reservoir or Forebay of his said fulling mill, Mytholm Bridge.