The Walbrook is one of the most important rivers in the world, which is pretty impressive for a river that no longer exists. It's not as important as the Thames, obviously, but the spot where these two rivers met defined the nucleus of a great global city. The City of London emerged along the line of this shallow river valley rolling down to the Thames, then spread across the contours of two low hills rising to either side. Drinking water, defensive position, strategic location, perfect. Those two hills still exist, with St Paul's Cathedral marking Ludgate Hill to the west and the Royal Exchange atop Cornhill to the east. But the Romans wouldn't recognise the valley which once divided their walled city into two halves, because the sparkling river has irreversibly vanished.
The Walbrook is therefore incredibly difficult to follow. You'd think it would be easy, passing as it did through one of the most well-documented square miles on the entire planet. But this means it disappeared early, several centuries before any other London lost river bit the dust. What had once been a "fair brook of sweet water" had by the 13th century become an ugly sewer that was "neither fair nor sweet". The middle and lower reaches of the Walbrook were paved over in 1463 thanks to a hygiene-minded Royal Act, and the original watercourse hasn't been seen since. Few accurate maps of the area were drawn up in medieval times, and the landscape has been built upon and built upon and built upon over the intervening years. So the account I'm publishing over the next few days is based on historical scraps I found in books, some approximate published maps and a bit of guesswork.