In message , at 06:14:30 on Thu, 7 Jan
2016, Roland Perry remarked:
The Great Ouse is tidal up to Brownshill Staunch, about 4 km upstream
of Earith. Though curiously (to the uninitiated) it's non-tidal
downstream of Earith.

That'll be Hermitage Lock, hiding in plain sight:

https://goo.gl/maps/YKR6xFHVrYq

...for the Great Ouse via Ely. The Environment Agency deems that the
Bedford Rivers are also part of the Great Ouse though, so that bit of
the downstream will be tidal. But it's very hard to see any tidal
component to the levels recorded here, which is about halfway to the sea
from Earith:

http://apps.environment-agency.gov.uk/river-and-sea-levels/120730.aspx?stationId=6245

As opposed to the next measuring point north of the

http://apps.environment-agency.gov.uk/river-and-sea-levels/120731.aspx?stationId=6290

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".
--
Roland Perry