5. The Rivers and Tributaries
A lot is made of the various personified tributaries of the Thames, split male and female at Teddington lock, where (as you head downstream) the river becomes tidal. Did you find these Orisa figures interesting? Credible? Important to the plot? There is a kind of two-way flow of appearance and character between the tributaries and the current human population of the area concerned. Did this work for you? In passing, CS Lewis calls his planetary tutelary spirits Oyarsa in his science fiction trilogy - an odd coincidence, though Lewis derived his word from Medieval Latin/European sources and Ben Aaronovitch from Nigerian.
For those wanting a bit more information, you could do worse than check out the following: https://en.wikipedia.org/wiki/Tributaries_of_the_River_Thames, https://en.wikipedia.org/wiki/Subterranean_rivers_of_London or https://londonist.com/2014/08/how-londons-rivers-got-their-names

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Comments
I liked the idea in concept. I would have been more convinced had they been ancient beings - maybe more along the line of the deities we saw in American Gods, which would certainly have been a home, here. I think it would have been very cool to see if the river personalities reflected those of the modern districts they flowed through. If the author did this in the novel, though, it was lost on me. He would to have had to call this out, specifically, for me to get it.
I also liked the concept of the rivers having rivalries - but I wasn't really convinced by some of the river gods themselves, how they became gods, or why the disagreed with one another.
So I guess the upshot is that I liked the concept, but not so much the execution of it.
I liked the concept, and I liked the execution of it. I thought it was interesting that the river spirits were eternal, but their human shells were temporary. It mirrored how the spirit of Punch took over human victims.
I also liked how the human vessels reflected the rivers themselves. Father Thames is the crotchety old bugger, connected deeply to the working-class rural England. Mother Thames reflects the cosmopolitan, global London based on trade and immigration. Tyburn was the gentrified region of London, rich and respectable, but with the undercurrent (pardon the pun) of the vicious public mass hangings.
Saying that, I don't think that the river spirits really added much to the story. It was interesting background story, but I don't feel that it was at all connected to the Punch plot.
Did they mesh well with the Punch plot? No, not at all, and I don't think they were intended to, or even should have. I like it better that they were bothersome but interesting intrusions on the main plot. It felt far more real to me that way.