It is hard to live in London and not occasionally feel haunted by the ghosts of its vanished rivers. For the past few years, my wife and I have been tracking them down. We have tramped along many trails in our time – from the Camino de Santiago to Offa’s Dyke – but there is a particular pleasure to be had in meandering across the streets of the capital in obedience to the whims of an invisible watercourse. It may seem odd to rate Croydon or Bermondsey above the Pyrenees or the Black Mountains; but such is the magic of a river as it flows beneath concrete and tarmac. Beauty is discovered in ugliness; mystery in the seemingly forgettable; wonder in the mundane.

Some rivers, of course, have left a more enduring mark on the city than others. Anyone who has crossed Fleet Street will know that. For those who know where to look, traces of a time before London swallowed up the water meadows that once extended north and south of the Thames are still visible in the twenty-first century: in street names; in the curving of valleys; in the rushing of waters dimly heard through metal grates. To follow their line is to journey back in time, and explore the contours of the landscape that existed before the city. The thrill is that of a treasure hunt: discovering in the curve of a railway concourse or in the jutting of a stink-pipe hints of a vanished geological age or an urban legend.

Some rivers, however, have vanishd so utterly that even their names are forgotten. Saddest of all, perhaps, is a stream that runs three miles from Stepney to Limehouse, but which ended up so foul and toxic that it came to be known simply as the Black Ditch. What might it originally have been called? In 1913, an antiquarian claimed that the Black Ditch had once been named the Barge River, and cited ‘old records of place names in the parishes and hamlets along the Thames side’ as evidence – but these, if they ever existed, have long since vanished. A map of 1746 features a riverlike squiggle next to what is now Mile End Stadium, and back then was still open fields; but does not provide a name. Yet once, before it became a sewer, the Black Ditch may conceivably have been navigable. That, at any rate, is what the etymology of Stepney suggests: for Stybbanhyð – first mentioned around AD 1000 – means in Old English ‘Stybba’s landing place’. It conjures up romantic images: of a Saxon adventurer sailing upriver from the Thames; of ships moored beside the Mile End Road; of round-houses planted beside what is now Stepney Green. One can, after all, but dream.

Which said, the weather on the day we did our walk was far too cold for dreaming. It was 5^th January: my birthday, but also the first day of the lockdown. The streets were empty. The effect was eerie. The trail of the phantom river itself seemed haunted by ghosts. Some, men who had owned manors in Stepney, were grand: St Dunstan, a 10^th century reformer and archbishop of Canterbury, famed for having clamped a pair of tongs onto the Devil’s nose; John Colet, a dean of St Paul’s who played host to Erasmus; Thomas Cromwell, Henry VIII’s most formidable minister. Most of the ghosts, however, were not grand at all. There were the poor who in 1832, when the Black Ditch was fetid with cholera, complained to the authorities that its waters were ‘liable to be choked up and the waters thereof rendered stagnant from dead animals, offal, broken vessels and other materials of an offensive kind.’ There were the rats who – according to the Booth Report in 1890 – were ‘the size of cats’. There was the White Horse, a pub demolished in 2003 that had stood for 300 years next to what originally had been a pond fed by the Black Ditch, and of which the only surviving trace is the statue of a white horse on a pillar. Of the Black Ditch itself there is barely a hint. Compared to the Fleet or the Tyburn, it is as though all physical traces of the river have been systematically erased.

Only at the very end of the walk does it suddenly make a dramatic appearance. The Limekiln Dock, where the Black Ditch joins the Thames, is a relatively short expanse of water that, at low tide, empties so completely that nothing is to be seen there but mud. The lime kilns that gave the dock its name are long gone, there is not a trace of so much as a tiny skiff, and the mud is dotted with litter; yet here, even so, at journey’s end, the Black Ditch manages to claw back some dignity. Perhaps there is even a hint of what it might have been before it became a sewer. Almost gone and forgotten it may be – but not entirely so.

The Black Ditch lives.