When the priests carrying the ark stepped into the Jordan, the water coming down from upstream stopped and rose up in a heap Joshua 3:15–16. Israel crossed over on dry ground while the priests stood firmly in the riverbed Joshua 3:17.
As Israel breaks camp to cross the Jordan, the priests carrying the ark lead the way. The moment their feet touch the river’s edge—at a season when the Jordan is flooding—its flow stops. Water coming from upstream piles up far away near a town named Adam, while the downstream flow toward the Salt Sea is cut off. This scene sits in the opening movement of Joshua, where leadership shifts after Moses and Israel begins entry into the land. In Joshua 3 the focus is on the ark leading the people and the careful ordering of the crossing, building suspense toward the moment the river gives way.
These verses present the Jordan crossing as a public, ordered event where the ark goes first and the priests carry it at the front (explicit). The turning point is timed: the water stops only when the priests’ feet touch the river’s edge, and the narrator stresses that this is flood season, when crossing would normally be hardest (explicit).
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Joshua 3:15-16, then read the surrounding section so the answer stays connected to the passage's flow.
15and when those who bore the ark were come to the Jordan, and the feet of the priests who bore the ark were dipped in the brink of the water (for the Jordan overflows all its banks all the time of harvest,)
16that the waters which came down from above stood, and rose up in one heap, a great way off, at Adam, the city that is beside Zarethan; and those that went down toward the sea of the Arabah, even the Salt Sea, were wholly cut off: and the people passed over right against Jericho.