Norbury is quick to state that the idea for her project comes from the out-of-print 1951 novel , by Scottish writer Neil M. Gunn, who walks alone into Scotland's wild places in search of - and ultimately finds - a well at the world's end. But no one knows where that well is and, as she's gifted away her own copy of the book, she uses another of Gunn's novels, , as a guide for her own narrative.

Taking its title from 's central metaphor, a structure that allows the annual migration of fish around an obstacle such as a dam, 's narrative is shaped around chapters that bear the names of rivers and places. Norbury's luminous observations of the natural world reshape and reinvigorate all notions of the places associated with those rivers, be they the Humber or Mersey, Skell, Severn or even Font del Mont, the spring near Barcelona where she and her novelist husband, Rupert Thomson, live for part of each year.

Many of these rivers and places, too, bear the variant Celtic names, be they Scots, Irish or, in the main, Welsh such as Afon Geirch, Ffynnon Fawr or Madryn. Norbury's beloved holiday cottage, a wedding gift from her husband to anchor her wandering spirit, lies on the Llyn peninsula in northwest Wales, so her knowledge of its terrain, language and lore is intimate.

Norbury's narrative also journeys into the meaning of the old words and ways, and the myths that gave rise to them. The river Boyne, for instance, owes its name to the mythical goddess Boand, who comes from a tale in the Irish Metrical Dindshenchas. According to one version of these ancient legends, Boand went to seek out a well beyond her own land. All were forbidden from approaching the well except the gods charged with its care, so Boand was punished by the well's water, which followed her, drowned her and kept on flowing. It became known as the river Boyne, thus preserving Boand's name, but 14 other rivers were said to come into existence when she upset the waters of the well, including the Tigris, the Jordan and the Severn.

This sense of ancient roots tugging at the contemporary British identity ripples subtly through the book, much like Norbury's yearning to uncover the mystery of her origins, which gathers unexpected momentum late in the narrative, turning this unusual work into a mystery as much as a memoir, and meditation on nature and belonging. Besides powering the narrative flow of the book, the theme is handled with raw honesty, narrative skill and eloquence.

It's as if, too, at times, these ancient places through which she journeys dislodge primal memories as well as ever more acute yearnings.

While in Liverpool, for example, she reveals how she was drawn to the mouth of the Mersey by her initial curiosity in Antony Gormley's famous sculptures, consisting of 100 cast-iron body forms facing out to sea. Here, in the landscape, after turning away from the sculptures, she experienced such a pull of recognition that she'd asked if there was a hospital nearby.

Directed to a 100-year-old Sisters of Mercy convent nearby, which used to be a hospital, she knocked on its door asking if it was possible she was born there and whether she could look at their records, only to find the nuns knew who she was but not the identity of her parents. "I was born in the convent and had then gone out into the world." She returns to the convent with Evie and her adoptive mother, where they say the rosary together, but her thirst for the truth of her own origins remains undiminished.

Another sojourn in Scotland's Grantown-on-Spey unlocks childhood memories of holidays with her adopted family, who had nurtured her love for nature and who she writes about vividly in . A visceral grief over the loss of her adoptive father, coupled with that over her lost baby as well as an inexplicable sense of foreboding, seep into the narrative.

Literary references too stain its pages in rich and varied hues, but never in a pompous or studied way. Instead it's as if fragments of poems, passages from books and even popular songs, well up from her memory in response to place.

The confluence of certain places and remembered words are a lightning rod for Norbury, dragging her into deep emotional eddies, rather like the dubh lochs, or bogs, that she was warned not to step into. Marked on Ordnance Survey maps as "blue spots, half the size of sequins, these treacherous tiny bogs, otherwise known as 'black waters', are sometimes only a few feet across", yet can suck a person down within seconds, as Norbury later discovers while on a pilgrimage to the source of the river Severn. But by then, her life has been sucked into an even more treacherous bog, brought on by a confluence of an entirely different order.

All the foreboding that seeps into the first part of her two-part narrative finds shape and voice two years into her watercourse project, when the global economic crisis leaves her and her husband in financial trouble and they are forced to return to England. At the same time she is diagnosed with a rare form of breast cancer and her adoptive mother falls seriously ill. But by then, what had started as a diversionary summer project now lingered as a habit.

"The rivers had evolved into a metaphor," she writes. "The waters became my guide, companion and teacher. They marked a border between different states of being: solid, liquid, air."

Fittingly, it is the Thames that gives her the strength to defy the odds. While explaining to a pair of disbelieving tourists one day that tidal flows make the Thames run upstream, at certain times of the day, she is struck by the thought, that "if the mighty Thames could reverse its direction twice each day, then surely it was possible that I could survive my cancer".

By then, the pressing need to be able to pass on her familial medical history to Evie has led her almost to her birth mother's door. Don't expect a fairy-tale ending, although a surprise lies in store in its final chapter. Rather, is an enchanted book that drives home the relationship between human narrative and the natural world, and the life-giving importance of both to the human soul.