When God told Joshua to have a man from each tribe pick up one stone and carry it to the place where they would camp that night, it was in part so that each tribe would have skin in the game of this memorial. A father could point out to his sons, "That's our stone, boys." It was personal to them. By the time they arrived by the Jordan River, forty years had passed, a whole generation had died, and Moses was no longer with them. Their track record was well established and it wasn't great. If God did something good for them, they were going to forget about it soon enough. So this time, God had them approach what he was about to do for them next in a different way, marking the event uniquely and wonderfully. Picture this. All of Israel is camped on the banks of the Jordan River across from the land God has said he will give to them. They take a good, long look at that river and they don't like what they see. It is full-sized, raging river, not a stream. I can hear them now. "Joshua, are you serious? How are we going to get to the other side of that? What kind of promised land is it if you can't even get to it?"
Well, that is what I imagine they said ...it certainly sounds like something I would say under similar circumstances. But God turned away from their murmurings. He told Joshua to instruct the priests to take the Ark of the Covenant -- the vessel that contained the tables of the Ten Commandments, symbolizing God's throne here on earth -- and to carry it out into the river and to wait and see what he would do. Joshua sent people throughout the camp to explain what was going to happen the next morning and to tell them to purify themselves because God was going to do great wonders among them. And they did. The next morning, the priest lifted up the Ark of the Covenant on their shoulders and took a few steps out into the river. And once again, just as the Red Sea had done for their parent' generation, the waters parted. First the women, children and elderly crossed over the riverbed and stepped onto the land on the other side. Then, before the army crossed, God commanded Joshua to have one man from each tribe to go into the river bed directly in front of the place where the priests were standing and to pick up a large stone, place it on their shoulders and carry it to the place where they would camp.
So the twelve men did exactly that. Now, the Bible doesn't tell us anything about this process, but I think we can safely assume that these guys didn't pick up little rocks. I've been around enough guys in these kinds of situations and I can't help but think that there was a bit of healthy competitition going on. After all, they had to represent! They were going to show that their tribe was the best tribe. And how could you tell? By the size of the stone each one hefted to their shoulder and managed to carry all the way across, that's how!
Once they were done, Joshua instructed that twelve more stones be piled up in the middle of the riverbed. Then the army crossed over and finally the priests carried the Ark of the Covenant across and onto the shore of the land God had given them. When the last sandaled foot of a priest stepped out of that riverbed, the waters that had been held back were released and filled the dry area again.
God had a plan in place to help them remember. He directed them to take those twelve stones and stack them upon each other as a memorial so that in the future when their children asked them about this pile of rocks, they could tell them, "They remind us that the Jordan River stopped flowing when the Ark of the Lord's Covenant went across. These stones stand as a memorial among the people of Israel forever so that we never forget the faithfulness of the Lord." Joshua 4:6,7