“That this may be a sign among you, that when your children ask in time to come saying, ‘What do you mean by these stones?' Then you shall say to them, that the waters of Jordan were cut off before the Ark of the covenant of YHWH. When it passed over Jordan the waters of Jordan were cut off, and these stones shall be for a memorial to the children of Israel for ever.”
The stones were intended for a permanent sign to future generations, to remind their children, and their children's children, of this amazing event. They would be able to stress that the stones came from the bottom of the river when it ceased flowing at YHWH's command. Note the repetition to bring home to the hearers the important and central fact, ‘the waters of Jordan were cut off'. Repetition was like two witnesses, it stressed the truth that was stated.