About 20 years ago, alas in the BW days, someone decided to demolish the wall between the GU and a short Stratford Canal pound. writes Mike Todd.
A great idea to save boat moves but it immediately allowed the Stratford to suffer water shortage if the Grand Union level was off.
Turned into a stop lock
The protection of the Warwick & Birmingham water was provided by the lock on the direct link below both 20 and 21 which was turned into a stop lock around 1816/1818.
The barrel roofed cottage seems to have been provided for a special lock man whose job was to ensure that the legally agreed levels were maintained. (see https://www.stratfordcanalsociety.org/junctions for more detail.)
The link goes back to the early days when the South Stratford was constructed, originally when all traffic from the north went on what is now the GU the basin, now below 20/21 level, was was the upper level and only reduced when the South Stratford was added and Lock 21 added.
Legal agreements were meant to limit the use of water from GU by the Stratford and extended court actions were taken to enforce this!
The indentations for the gates can still be seen clearly in the link lock chamber.
Through route
The stop lock was made into a through route around 1966 it seems, when ownership of water was not in itself an issue—the question of water management is a different issue—when BW was three years old but now 'owned' both canals. (My guess is that BW at the time saw a benefit in removing the maintenance cost of the stop lock gates.
However, it also had the advantage that passage from GU to South Stratford was much easier.