Canals in England are fascinating. They are man made from the efforts and hard work of a few dedicated men. In the Industrial Age they were a vital factor in the prosperity of many regions of the UK.
This is an old picture of a lock which was still in use connecting parts of the canal. The traffic at that time was purely "pleasure boats". I think it was in the West Country somewhere, so maybe it is the Brigewater Canal.
When you stand on the towpath it’s hard to realise that there has to be a water source to supply the canal. So I guess there must be a very slight gradient over lengths of waterway to enable the canal to function. Any boat entering a lock must have some effect on the water level which must be compensated for from a river somewhere.
This balance between level and flow must have taxed the ingenuity of those early engineers and one can only guess at the detailed measurements and calculations which were needed.