The race started at 9:10 AM but the 10k portion of the race had over 9,000 people so it took at least 30 minutes to get everyone through the starting gate.
The 5k portion had thousands more and it took quite a while for us to get over the starting mark. Just think if it had been over 2 million people, like the number of people who crossed the Jordan one day over 3,000 years ago. Joshua was the leader of this large group of people. The people crossed on dry land. The "ford" was bigger than the starting gate of a 10k race but still fairly small, I guess, when you consider how many people had to get across. It took a long time for everyone to cross over. They crossed before Joshua and before God, and before the Ark of the Covenant which was held by priests in the middle of the river until all had passed over.
After they all passed over and while the priests were still in the riverbed God told Joshua to send men back, one from each tribe to go back to the river to each get a large stone out of the river bed. Each man got one large stone. They carried them to the place when the new nation was to camp for the night. They used these rocks to make a memorial to what had happened that day. When in the future people had a hard time believing that anyone could have crossed the Jordan here (on dry land) they could point to the pile of rocks. They passed over in the harvest season, the time when the Jordan would be at flood stage. This was the time of year when the Jordan was strongest. Someone could point to these smooth stones, obviously coming from a river bottom and say, "Here is proof!"
We are sometimes tempted to think that people in earlier times were more gullible, more superstitious than they are now. But not really. There is no reason to think so. People have always trusted their experience. If in their experience a thing was unlikely, even impossible, they are not going to readily believe you when you say it once did happen. Crossing the Jordan was miraculous. If you did not see it with your own eyes, you are not likely to believe it happened. No matter how much you trust the person's testimony, you are still going to doubt. So the stones were set up to help people believe. So God had Joshua put up a memorial.