Crossing running water is a remedy against supernatural entities. According to widespread folklore beliefs, water is pure and holy and will reject the unholy. A person threatened or pursued by ghosts, fairies, demons, witches, vampires, and so forth can halt them in their tracks by crossing running water.
During the witch hunts of the Inquisition, one method of determining the innocence or guilt of accused witches was “swimming,” in which they were tossed into deep water with their arms and legs bound. If the water rejected them and they floated, they were guilty. If they were innocent, the water accepted them and they sank. In either case, the witch usually was doomed. Sinking often meant drowning. Floating was a certain sentence to execution by hanging or burning.