4:1-9 The works of the Lord are so worthy of rememberance, and the heart of man is so prone to forget them, that various methods are needful to refresh our memories, for the glory of God, our advantage, and that of our children. God gave orders for preparing this memorial.

Jos 4:9. Twelve Stones Set Up in the Midst of Jordan.

9. Joshua set up twelve stones … in the place where the feet of the priests … stood—In addition to the memorial just described, there was another memento of the miraculous event, a duplicate of the former, set up in the river itself, on the very spot where the ark had rested. This heap of stones might have been a large and compactly built one and visible in the ordinary state of the river. As nothing is said where these stones were obtained, some have imagined that they might have been gathered in the adjoining fields and deposited by the people as they passed the appointed spot.

they are there unto this day—at least twenty years after the event, if we reckon by the date of this history (Jos 24:26), and much later, if the words in the latter clause were inserted by Samuel or Ezra.