This is one of the few signs left of the River Avon navigation between Salisbury and Christchurch. The remains of the lock are on a manmade cutting that runs from Harnham to Longford castle. Work on the navigation started in 1675 and it opened in 1684. There are records of barges operating between Salisbury and Christchurch, however the navigation was abandoned in 1730.
The brickwork and basic structure of the lock are in surprisingly good condition considering it's age, although there have clearly been remedial works carried out over the years. The wooden lock gates are long gone, the top gates would have been where the weir is today and the bottom gates at the upstream end of the modern...