As soon as the Jewish people crossed over the River Jordan, Hashem tells Yehoshua to choose 12 men, one man from each tribe, and tell them to take a stone and set it down in the place where the Jewish people had just encamped.
Yehoshua tells the 12 men to take a stone each, carry it across the river and set it down as a monument to the fact that the Jewish people had just crossed the River Jordan. The Kohanim kept standing in the river to let the Jewish people cross over safely.
When everybody was safely on the other side, Hashem told Yehoshua to tell the Kohanim to come out of the river. Just as the Kohanim ascended from the river, the waters of the river returned to normal and flowed freely as beforehand.
The people crossed the River Jordan on the 10th of Nissan and encamped at Gilgol. This is where Yehoshua erects the 12 stones, which they had taken from the river.