An extract from Ghost Trees: Nature and People in a London Parish by Bob Gilbert, published this week by Saraband (hardback, £14.99).

The source of our little, sometime stinking river might be just a ditch, but it was a ditch over which Shakespeare had stepped, and Amy was keen to locate it. Holywell Street seemed a good start and dowsing along it led us into a car park. An attendant came out of his hut to ask if we needed help but we reassured him by telling him we were dowsing the Black Ditch. He walked off without a further word, as if this were a regular occurrence. One of the walls surrounding the car park had been extensively covered in colourful graffiti and bore, in large, flowing letters, the legend ‘End of the Line. Rock Well’. I have no idea what it meant but for Amy it was a sign. It was also an early indication of the way that she would see things, her imaginative antennae attuned to any incidental connection or tangential reference to water. She seized excitedly on the name of Waterlow Buildings, of Salmon Lane and Hare Marsh. It was surely not coincidence that at Tapp Street a roadside tree hung over the street sign, obscuring the second p. There was, too, a beautiful old building that had clearly once been a bath house. Built in bright red brick with white stone courses and decorative semi-circular windows, it had tall chimneys rising from a black slate roof. The separate entrances were still signed with ‘Men’s Bath’ and ‘Women’s Bath’ and beside them stood sculpted semi-naked figures draped only in towels.

Appearances of the word ‘black’ were just as important. Outside Blackman’s Shoes she asked the bemused owner if his cellar ever flooded while later, beside a street tree, we came upon the most curious of our finds. The tree was a white poplar, its shed leaves littering the pavement around us, anaemic greeny-yellow on the upper surface but still a felty white beneath. Discarded in the pit below its trunk was a pile of discarded CDs labelled ‘Noir Music’. I took one home and am playing it as I write. It is electronic dance music, insistent and repetitive, and a couple of tracks will probably suffice.

All of this had an unintended connectivity, a synchronicity that the surrealists would have loved. ‘It was,’ Amy told me, quoting Beuys again, ‘all about the plurality of experience.’ These were evidences from the imagination, cabalistic clues that we were on the right track. And, by and large, we were. This, I have to say, was all to do with Amy’s metal rods and nothing to do with my hazel. Try as I might, I could get nothing from them, and my English and Turkish sticks were equivalent in their ineffectiveness. Occasionally a passing lorry deceived me into detecting a tremor but any other communication was disappointingly absent. It would be wrong, however, to blame the hazel. Dowsing may be dismissed by official opinion as a ‘pseudo-science’ but the fault in this instance was almost certainly mine. Perhaps I had cut the sticks in the wrong way or in the wrong place or at the wrong time. Perhaps the persistent drizzle was a form of static interference. Or, most likely, I had approached the whole thing with too much of a doubting, left-brain logic. Clearly my knees were not doing enough of the thinking.

We must have made a strange sight walking in the rain through the East End streets, holding our divining rods ahead of us; me with a forked length of hazel, Amy clutching her rods in tight-lipped concentration. Periodically she would stop to ask her silent question and then take off again, pacing forward or veering in a completely different direction. At a hot tea stand, workers with bacon rolls and steaming mugs took a particular, and bemused, interest in what we were doing. Groping for an answer I suggested that we were employees of Thames Water and that this was our latest technology, an assertion that they seemed to have no problem accepting.

But did it work? As we walked we seemed to deviate regularly from what I remembered of the ‘book’ route but just as regularly we would come back to it again. As we veered in different directions I would bite my tongue, but before my concern could get the better of me I would find us veering back again. We created a much more meandering river, I suspect, than the real one; zig-zagging around it and adding curious curves, and in Poplar the rods seemed committed to a short cut, omitting much of the loop in which the river was supposed to have run. And yet we somehow found ourselves at Stepney Church and the second source at Rhodeswell, and later still, at Limekiln Dock, the undisputed mouth of the river. Here, where a cul-de-sac of a creek abuts the Thames, we stood by the dock wall and looked down. The tide was out but immediately below us a little sluice was emptying a trickle of water onto the mud and gravel. ‘And there,’ said Amy, ‘is your lost river.’

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Ghost Trees: Nature and People in a London Parish, published by Saraband, is out this week and available here.