I'm on my twelfth or thirteenth read of LotR and something finally clicked that I can't unread. I've written it up properly on Medium (link at the bottom) but I wanted to bring the core argument here because this sub is where it'll get the scrutiny it deserves.
The short version: I think rivers in Tolkien function as a coherent theological system, not just atmospheric geography, and Ulmo's refusal to leave Middle-earth is the key.
Ulmo stayed. Every other Vala retreated to Valinor. He remained, speaking through every river and stream. The Silmarillion is explicit about this. Once you hold that in your head, every river crossing in LotR starts to feel different.
The Bruinen doesn't just defend Rivendell tactically. It refuses the Nazgûl. The Anduin receives Boromir's body gently and carries it toward the sea, toward Ulmo's fullest domain, and it seems to know the difference between his failure and his redemption. Frodo and Sam crossing the Anduin alone is the true end of the Fellowship, and the river marks the threshold.
The part that really got me was tracing the Nimrodel chain. Nimrodel flows into the Silverlode, the Silverlode into the Anduin, the Anduin into the Bay of Belfalas. And Amroth drowned in that bay searching for Nimrodel. Two voices in the same river system, moving toward each other for three thousand years. The river remembers.
Then the inverse: the Enchanted Stream in Mirkwood steals memory where Nimrodel carries it. The Dead Marshes trap the dead in a grotesque inversion of the same function. And Gollum, the most corrupted creature in the story, is the one who navigates them. Rivers under Ulmo's care carry, heal, judge, and remember. Rivers under shadow invert every one of those things.
And then there's Saruman damming the Isen. The Ents don't defeat him primarily through force. They restore the river to itself. That feels less like military strategy and more like something liturgical.
(EDIT: I know that is not actaully what happens. My brain is still convinced there is a line in Flotsam and Jetsam where it explained that that the Isen flows naturally flows through the yard of Isengard before Saruman had it dammed. And Merry & Pippin explain in the guard house that the Ents break the dam.)
All of which led me to Goldberry. If rivers are Ulmo's continuing voice in the world, the River-woman's daughter isn't just a nature spirit. She might be the most direct remaining embodiment of that divine presence in the living world. And Tom, if you read him as the spirit of Arda itself, loving Goldberry starts to feel like the world knowing itself through its own waterways.
I'm not claiming Tolkien consciously designed all of this as a system. But I think the internal consistency of his world is deep enough that the theology emerges whether it was explicitly placed or not.
The full essay with the complete argument is here: https://medium.com/@frimodig/rivers-in-tolkien-are-not-geography-i-think-they-might-be-theology-b3da9625f44d
I'd genuinely love to know what I'm missing. There are river moments I'm sure I haven't traced. What would you add?