Joshua - Chapter 4
The Fulfillment of a Rare Mitzvah
At this juncture, Yehoshua erects a memorial to commemorate the miracle that Hashem performed for the Jewish people at the Jordan River. He ordered twelve men, one from each Tribe, to each carry a stone from the river to the place where they were camped. This was actually the fulfillment of a commandment given in Deuteronomy 27, in which the Jewish people were commanded to erect 12 stones and to write the Torah upon them.
The entire time, the Kohanim had been standing there, holding the Aron. (Actually, the Aron carried its bearers, but that's another story.) When the people were done erecting the stones, the Kohanim carried the Aron out of the river bed and the Jordan resumed its usual flow.
The people set up camp in Gilgal, at the eastern end of Jericho. Gilgal would serve as the temporary capital for the next 14 years.