In message , at 12:14:26 on Thu, 7
Jan 2016, Clive D. W. Feather 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".

Don't know.

The OS 1:50,000 maps show the Great Ouse as tidal from the sea to
Denver, then non-tidal to Earith, then tidal to Brownhill. The New
Bedford is shown as tidal throughout; the Old Bedford and Delph as
non-tidal throughout. The boundaries are all things like sluices.

One of the reasons I'm not convinced that the New Bedford is tidal in
the sense of going up and down twice a day is that when it floods at
Welney the water doesn't flow away to the Wash for weeks.
--
Roland Perry