A man had landscape workers put down 60 linear feet of river rock, including a felt bottom, to strengthen a small drainage ditch. They did a good job. After several weeks went by, he checked on the drainage ditch, and it appeared that someone moved about 10 feet of rock downstream leaving a bare spot. He put the rocks back in their original position, removing the bare spot. Three weeks later it happened again, leaving still another bare spot.
Question: Can you explain how this happened?