I think that's the Westbourne - it flows in a giant green cast iron pipe over the platform at Sloane Square tube. the Walbrook is really lost now - it's in the square mile (near but not under the street called Walbrook) - it was regarded as a 'sacred' river - by the Romans and possibly earlier - that's where they found the Temple of Mithras.

The Tyburn flows down near Marble Arch (previously known as the execution site called Tyburn cross) next to Park Lane, splits and empties into the Thames in two places - originally making Westminster an island. Then there is the Neckinger, the Peck, the Ravensbourne, the Effra, the Wandle, Deptford Creek, Stamford Brook (you've heard of that right?) etc, etc.

Sorry this is off topic and I am being a bore

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