Myths and Legends of our Own Land, by Charles M. Skinner, [1896], at sacred-texts.com
Above Georgetown, on the Potomac River, are three rocks, known as the Three Sisters, not merely because of their resemblance to each other—for they are parts of a submerged reef—but because of a tradition that, more than a hundred years ago, a boat in which three sisters had gone out for a row was swung against one of these rocks. The day was gusty and the boat was upset. All three of the girls were drowned. Either the sisters remain about this perilous spot or the rocks have prescience; at least, those who live near them on the shore hold one view or the other, for they declare that before every death on the river the sisters moan, the sound being heard above the lapping of the waves. It is different from any other sound in nature. Besides, it is an unquestioned fact that more accidents happen here than at any other point on the river.
Many are the upsets that have occurred and many are the swimmers who have gone down, the dark forms of the sisters being the last shapes that their water-blurred eyes have seen. It is only before a human life is to be yielded that this low wailing comes from the rocks, and when, on a night in May, 1889, the sound floated shoreward, just as the clock in Georgetown struck twelve, good people who were awake sighed and uttered a prayer for the one whose doom was so near at hand. Twelve hours later, at noon, a shell came speeding down the Potomac, with a young athlete jauntily pulling at the oars. As he neared the Three Sisters his boat appeared to be caught in an eddy; it swerved suddenly, as if struck; then it upset and the rower sank to his death.