In message , at 11:40:10 on Thu, 7
Jan 2016, Richard J. remarked:
I wonder if there's a finesse in the expression "tidal", which means "up
to the first lock", rather than "the part of a river whose level is
affected by the tides".

The Thames is generally held to be tidal as far as Teddington Lock, but
sometimes the high tide manages to go further, and the flow gauge at
Kingston (upstream of Teddington) gives negative readings.

Do they leave both sets of lock gates open?
--
Roland Perry