“If this is hard to understand from a map, the rest is harder. For one thing, the river that flows ever onwards is also seeping sideways, irrigating the fields and land to one side and the other. It finds its way into wells and is drawn up to launder petticoats and be boiled for tea. It is sucked into root membranes, travels up cell by cell to the surface, is held in the leaves of watercress that find themselves in the soup bowls and on the cheeseboards of the county’s diners. From teapot or soup dish, it passes into mouths, irrigates complex internal biological networks that are worlds in themselves, before returning eventually to the earth via a chamber pot. Elsewhere the river water clings to the leaves of the willows that droop to touch its surface and then, when the sun comes up, a droplet appears to vanish into the air, where it travels invisibly and might join a cloud, a vast floating lake, until it falls again as rain. This is the unmappable journey of the Thames. And there is more: what we see on a map is only the half of it. A river no more begins at its source than a story begins with the first page.”