Despite being called Whitehall Lower Lock, this lock is really the top lock of a short flight of three that includes the two Puck Mill locks. It is one of those on the canal designed to take craft up to the standard dimensions of the Thames barges. It is thus a little over 90 feet long and about 13 feet wide. It had a rise of 8 feet. It was also one of the locks shortened in 1840-41 by about 20 feet by throwing an arch across the top end of the lock and resiting the cill and the gates. This was to reduce water consumption in the locks and was possible because by this time shorter ‘long’ boats had replaced the Thames barges on this section.
The chamber is mainly brick lined, though in poor condition, with a substantial stone coping and stone-edged recesses for the gates; it was probably originally stone-lined. There are two sets of recesses at the top end of the lock, one for the original gates and the other the result of the shortening of the lock.
In this lock, the approach at the top end suddenly narrowed, instead of gently funnelling. The ground paddles above the top gates were placed at right-angles to the cut against the end walls of the lock structure on either side of the approach. The chamber is spanned by a modern footbridge.
Despite repairs and patching, and the fact that it is heavily overgrown, the lock chamber is substantially intact.