On a rafting trip in early September, Christine Hutton, her husband, Richard, and a dozen of his corporate colleagues paddled to a flat, outcropped rock just a few miles from their push-off point at Jerry’s Three River Campground in Pond Eddy. The trip had been sluggish; the Delaware was at its lowest level in some 50 years, they’d been told. The group, in its triumvirate of rafts, stopped to have lunch. “One of our associates stepped out to a rock in the middle of the river, and picked up what looked like a stone tablet,” recalled Christine Hutton. It was...