St John's Lock, below the town of Lechlade, Gloucestershire, is the furthest upstream lock on the River Thames in England. The name of the lock derives from a priory that was established nearby in 1250, but which no longer exists. The lock was built of stone in 1790 by the Thames Navigation Commission.
The main weir is downstream, just below St John's Bridge, where the River Cole and the River Leach join the Thames on opposite banks.
A statue of Old Father Thames by Raffaelle Monti is outside the lock house.
Wikipedia https://en.wikipedia.org/wiki/St_John's_Lock
Coordinates 51°41'22.999" N -1°40'47.931" E