What a bloody good thread, Al. Something I have mused over meself, in a boaty kind of way. A few years back, I was staying with some mates near Milano, on the banks of a canal that yer code man Da Vinci designed, but the water charges down that at over six knots. I'm told it was built to deliver drinking water to Milano from the Italian Lakes, but that it was also used as a waterway. I can't believe that there's any locks on that stretch, as it was a downright hill.

Get back to us when you get some more griff on this. I thought as you did, that it was an IR thing, but them Dutch lads have been teaching us about draining fens a lot longer than that, and in South Ferriby, home to Benson's Tripe Works, Rugby Cement and a sometime contributor here, the River Ancholme is locked to keep the high tide out of the lower lying Vale of Ancholme, and to my certain knowledge, sea walls have been employed to reclaim farming land well before the IR, in the mid 18th century, so it is reasonable to suspect a crude lock would've been employed in such places, if just to repell high tides.