Buildsalz provides the only attestation of this Middle English word in an Anglo-Norman context. The word is not listed in the MED or OED, but compare MED bai n.4
(‘weir, millrace’) and OED bay n.5 (‘embankment or dam to retain water, or divert its course into a mill stream’).
s.
1geog.upper section of a weir, mill-race (i.e. dammed canal or embankment diverting water to a mill):
(
1387-88
)
ount empris de faire de novell deux molynes eweres [...], c'est asaver le corps et le upperbay joynaunt bien e covenablement a le rerebay ovesque tout la carpentrie q'appartient a ceo
468
upperbay (1387-88)