The New Murrey and Blue blog

We are all familiar with modern locks of the sort found on canals and rivers today. Two sets of gates, and a space between from which, or into which, water can be removed or added by mechanical means. Once the water level has risen or fallen (as required) the vessel can proceed.

Medieval (and early modern) locks were not like that at all. They were more like weirs. Sometimes described as “flash locks”, there were twenty six such on the Thames between Lechlade and London.

They had but one “gate” and even that is a simplification, as it was nothing like a modern lock gate. It was a set of boards, called paddles, supported against the current by upright timbers called rymers which normally kept the level of water above it to navigable levels. The effect of this was to build up a pool, often associated with a mill making use of the “head” of water so formed.

To pass downstream the “gate” was opened – exactly how is not clear, but it seems to have been an inherently hazardous business. The barge (or barges) would then “shoot the rapids” propelled by the sudden release of water. This had implications for the miller (if there was one) and ill-feeling developed between millers – who would lose their “head” of water – and bargees who wished to make progress.

Going upstream was even more awkward. The barge had to be towed against the current, either by manpower, horse power, or the use of a winch.

The locks on the Thames were usually kept closed – but there was a limit to the number of times they could be opened in a day without lowering water levels too much and without making Mr Miller very cross. This must have been an annoying restriction on river trade.

An interesting discussion on the subject may be found here.