Beneath the stone pulpits jutting from the sides of Blackfriars Bridge are J.B. Philip's birds, skilfully carved from Portland stone. On the east-facing pulpit they're seabirds; on the west-facing pulpit you'll find freshwater species.
It's a nod to how the salinity and wildlife of the Thames transforms the closer you head out to sea. But where exactly does the Thames change from fresh to salt water? Spoiler alert: it's not Blackfriars Bridge.
Somewhere else you might think the sea change (pun intended) might occur is at the mighty Thames Barrier. But again, you'd be wrong.
Truth is, the answer to this one isn't plain sailing.
Let's go to the source of the river — and even this isn't straightforward, because there are two credible Thames sources — one near Cirencester, the other a little outside Cheltenham. Meandering through Oxford, Reading, Henley, Marlow and Windsor the river then makes its Greater London debut at Hampton.
And officially, the tidal limit of the Thames is located not much further downstream, at Teddington Weir. Yet if you stuck a finger in the water at this point and tasted it (and by the way, you really shouldn't), don't expect it to be salty.
As observed on Floating Down the River, while the river does start turning saline after Teddington, because of its irregular, serpentine nature — which mixes together fresh and salty water — the Thames remains more or less freshwater all the way to Battersea. Even then, the water's only brackish, not saline, per se.
And though the river does gradually increase in salt content overall as it nears the North Sea, where tributaries flow into the Thames — at Creekmouth in east London, for example — the water is fresher, and less salty.
In short: most of the Thames within Greater London contains saltwater — but the river's not necessarily that salty.
Marine animals including seals, dolphins — even sharks — have been known to ply the Thames, and while they're usually spotted around the river mouth (where you'd expect to find them), they can swim up as far as central London — and even Teddington.
With thanks to David Whittaker for his help researching this article.