‘And so it was that when the people removed from their tents to pass over Jordan, the priests bearing the Ark of the covenant being before the people, and when those who bore the ark were come to Jordan, and the feet of the priests who bore the Ark were dipped in the brink of the water, for Jordan overflows all its banks all the time of harvest, that the waters which came down from above stood. They rose up in one heap a great way off at Adam, the city which is beside Zarethan, and those that went down towards the sea of the Arabah, even the Salt Sea, were wholly cut off.'
This long and complicated sentence describes what happened. The people struck camp, the priests then bore the Ark before the people, they arrived at the Jordan, their feet entered the water, and then it happened. The waters ceased a great way off so that where the river had been swollen and raging it became an empty river bed before their very eyes.
“The people removed from their tents to pass over Jordan.” They took down their tents and packed their possessions for the last time in Beyond Jordan. For they knew that somehow, although they did not know how, they were going to cross over. Then they marched two thousand cubits behind the Ark, led by the priests who carried the Ark, the Ark which witnessed to their covenant with YHWH, and by all the priests and Levites.
And the priests who bore the Ark marched steadfastly up to the swollen, fast moving waters of the Jordan and stepped into the waters at their edge.
Then ‘the waters which came down from above stood. They rose up in one heap a great way off at Adam, the city which is beside Zarethan.' Compare Exodus 15:8 whose language is reflected here. Adam was just over twenty eight kilometres (seventeen miles) north of Jericho, near Zarethan in the Jordan valley (Zarethan was probably on the west side of the Jordan), where there is a ford over the river. The river may have been blocked as a result of an earth tremor causing the collapse of high banks of the river and of cliffs by the Jordan. This would make a dam against which the waters would rise up in a heap. It would leave a twenty eight kilometre (seventeen mile) gap for the Israelites to make use of. Similar events are well documented as having occurred at times through history in, for example in 1267, 1909 and 1927. But this was at God's timing.
“And those that went down towards the sea of the Arabah, even the Salt Sea, were wholly cut off.” Once the waters stopped flowing from above, the waters below would subside, and eventually disappear into the Dead Sea leaving a further stretch of dry land for any number to cross. ‘The Sea of the Arabah' would appear to be the ancient name of the sea, which was later called ‘The Salt Sea'. The name ‘The Dead Sea' is post-Old Testament.
Notice how quietly the miracle is described. There is no fanfare. It is just assumed that the disappearance of the waters occurred because the priests bearing the Ark entered the waters (contrast Exodus 14:21), although later it will be made clear that it was YHWH Who did it (Joshua 4:23).