There are a series of waterways in London which are known as "The Bow Back Rivers"
There is a map of them here.
Bow Back Rivers
Logically however, if the gradient of a river is low, so that Oxbow curves and meanders are created, it would be a very poor place to put a mill, which needs either a head of water or a rapid flow rate.

As for gay and grinding, "grist and grinding" would hang on to the Mill-Wheel image, and since the rumour process often substitutes a familiar word for an unfamiliar one with little concern for meaning as seen the thread on "Eeny meeny macaracca" which acquires a lollipop
Cheers
Dave