"And it came to pass, when all the people had completely crossed over the Jordan, that the LORD spoke to Joshua, saying: 'Take for yourselves twelve men from the people, one man from every tribe, and command them, saying, "Take for yourselves twelve stones from here, out of the midst of the Jordan, from the place where the priests' feet stood firm. You shall carry them over with you and leave them in the lodging place where you lodge tonight."' Then Joshua called the twelve men whom he had appointed from the children of Israel, one man from every tribe; and Joshua said to them: 'Cross over before the ark of the LORD your God into the midst of the Jordan, and each one of you take up a stone on his shoulder, according to the number of the tribes of the children of Israel, that this may be a sign among you when your children ask in time to come, saying, "What do these stones mean to you?" Then you shall answer them that the waters of the Jordan were cut off before the ark of the covenant of the LORD; when it crossed over the Jordan, the waters of the Jordan were cut off. And these stones shall be for a memorial to the children of Israel forever'" (Josh. 4:1-7).
It appears that twelve men (one from each tribe) had been selected to cross the Jordan last (cf. Josh. 3:12). When they crossed, each one was to pick up a large stone (though not so large that one could not carry it on his shoulder). They would then pile up the stones at the encampment that night (Gilgal, specifically) as a memorial to God's great act of power that day. He enabled His people to cross the Jordan River on dry ground! This would be something their children would inquire about. They would want to know why the pile of rocks existed at that location, and this would be a wonderful opportunity for the parents to teach and review the history of God's people. Additionally, the rocks would serve as a memorial for the adults. When their faith in God was weak or doubtful, they could be reminded of God's omnipotence and care for them when they saw the rocks at Gilgal.
The twelve obeyed Joshua's command, but a careful reading of the chapter reveals that actually there were two piles of twelve stones that served as memorials. In addition to the one set up at Gilgal, Joshua "set up twelve stones in the midst of the Jordan, in the place where the feet of the priests who bore the ark of the covenant stood; and they are there to this day" (Josh. 4:9). The verse does not teach that the stones are still piled there today, but that they were still intact at the time of the book's writing (presumably a number of years after the events in this chapter took place). It is unknown how large these stones were or if they were visible during the harvest time where the river flooded. But, surely they were visible at certain times and would also serve as a reminder.
After all had crossed over as quickly as possible (cf. 4:10) and Joshua had arranged stones in the midst of the Jordan, the priests who carried the ark then came up out of the Jordan. "And it came to pass, when the priests who bore the ark of the covenant of the LORD had come from the midst of the Jordan, and the soles of the priests' feet touched the dry land, that the waters of the Jordan returned to their place and overflowed all its banks as before" (4:18). The miracle God provided ended, as we would expect, as soon as it was no longer needed. What a favorable impression this whole event should have made in the minds of the people. Joshua was their leader and God was with them! They were now ready to conquer the land they had entered.
Friends, we today need to respect and honor the divine customs and memorials given in the New Covenant for ourselves and teach them to our children (e.g., the Lord's Supper; cf. I Cor. 11:23ff), that we may never forget and that they may learn God's greatness and love! May we teach our children faithfully at every opportunity (cf. Deut. 6:6,7) or else a generation may tragically rise up that does not know the Lord or His great works (cf. Jud. 2:10)!