20 years ago I was in the middle of my obsessive walking project – journeying along the routes of now buried rivers in London – that became The Groundwater Diaries. Here’s the map of The New River that appeared in the book. “London is a city of invisible boundaries. Areas alter in atmosphere or architecture in the space of a few yards, and a reason for this might be that the rivers which once flowed were often the borderlines between ancient parishes and settlements. You might walk down a street now and suddenly notice a change in the air. Chances are you have walked across the course of an underground river. The New River would have been no different. Although a recent addition to the waterways of London (about 400 years old), when it was built it would have run through mostly open countryside and settlements would have grown around it.

Some portions of the New River are visible to the naked eye. Yet these sections (for instance, Turnpike Lane to Finsbury Park), which flow silently behind housing estates and terraced streets, seem somehow not as alive as those which have disappeared. It’s the ghost parts of the river, now covered by houses, gardens, shops, parks and roads, that get me going more than the algae scum cuts I can see filled with bikes, shopping trolleys and empty plastic Coke bottles.

Searching for lost rivers is, in a way, a spiritual journey, searching for things that I once valued but have lost…”