...it was about ten o'clock on the morning of the 1st of August when I found myself in Trewsbury Mead...[at a] cottage where I made final inquiries for the head of the river.
"Follow along the edge of the meadow," the old lady said, "till you come to some straggly bushes, but pay no attention to them. Follow along then," she said, "till you come to a dead tree, but pay no attention to that either. Follow along again," she continued, "till you come to a big tree, I don't know if it isn't an ash, and on that you'll see the letters T.H. There's a well alongside of the tree, an old Roman well, they say, and that's where the Thames rises. You can't miss it. T.H., that means Thames Head, and it's cut in the bark."
Sweet Thames Run Softly, written and illustrated (with beautiful woodcuts) by Robert Gibbins, published in 1940. A delightful commentary on rivers, nature, travel, folklore, and amusing encounters along the way as the author travels by home-made punt (which he actually rows rather than punting) down the Thames.