Tom Bolton is a writer, geographer and charter of lost rivers. Here is a short interview with him, about his work and the hidden waterways that course beneath London.
Thanks to Tom - here is a link to his blog, and to make sure you have a look at his book, London's Lost Rivers. It is a fascinating read. He's also working on a second volume at the moment, so do keep your eye out for that.
If you want to visit the mouth of the Walbrook, from Cannon St Station, turn left down Dowgate Hill, cross the busy road at the bottom and across to Cousin Lane. At the end, by a pub called The Banker, jump up the steps that lead over the embankment wall, and then down to the shore.