Josh 3:14 When the people left their tents to cross the Jordan, the priests carrying the ark of the covenant went ahead of them. 3:15 When the ones carrying the ark reached the Jordan and the feet of the priests carrying the ark touched the surface of the water—(the Jordan is at flood stage all during harvest time)—3:16 the water coming downstream toward them stopped flowing. It piled up far upstream at Adam (the city near Zarethan); there was no water at all flowing to the sea of the Arabah (the Salt Sea). The people crossed the river opposite Jericho. 3:17 The priests carrying the ark of the covenant of the LORD stood firmly on dry ground in the middle of the Jordan. All Israel crossed over on dry ground until the entire nation was on the other side.

This is one of three miracles in the book of Joshua (the others being the fall of Jericho’s walls in Josh 6:20 and the lengthening of the day in Josh 10:12-14) that critics tend to regard as legendary.

At the opposite end of the spectrum are those like Roger Ellsworth, who in his popular-level treatment of Joshua, regards the incident as pure miracle:

If we believe in God, we have no reason to doubt the miracles of the Bible. God made this world and put in place the laws by which it operates, but he is not shut out of his own world. He can, when it so pleases him, step in to suspend the very laws he has made and do that which is extraordinary.’ (Opening Up Joshua)

But it is no less honouring to God to suggest that, rather than suspending his own laws, he used them.

Colin Humphreys interprets this event as reflecting the ordinary processes of nature, where the miracle is that of the timing of the event.

Humphreys notes the rather precise geographical indicators given here.  In particular, he says, it is noteworthy that the text states that the water ‘piled up in a heap a great distance away’, and even tells us exactly where.  This itself counts strongly against the story being fictional.

As for the city called Adam, scholars are generally agreed that it was what is now called Damia (i.e. dropping the Hebrew breathing at the beginning of the name, retaining the two consonants, and adding an Arabic ending.  This town is some 17 miles north of Jericho, on the banks of the Jordan River.

It is to be noted that the River Jordan lies along the great rift valley, which is know for its frequent earthquakes due to tectonic activity.  For Humphries, a likely scenario is that an earthquake caused a mudslide upstream, leading to a temporary interruption in the flow of water.  The flow of the Jordan River has been cut off temporarily on a number of occasions (1096, 1160, 1267, 1546, 1834, 1906, and 1927).  These generally lasted one or two days.  The 1927 cutoff (which lasted about 21 hours) was caused, as the others probably were, by an earthquake leading to a landslide.  It is possible that the earthquake that caused the cutoff recorded here also led to aftershocks that resulted in the collapse of Jericho’s walls.

Instone-Brewer (Science and the Bible) adopts the same view, noting that:

‘Someone who knows the area would know that when the Jordan passes near the town of Adam (modern-day Damiya, eighteen miles north of their crossing), it bends around a steep slope of loose mud and stones. Every now and then a section of the steep bank collapses and dams the river; then, after several hours, the water breaks through and pushes the mud aside. This is a very rare event…but the possibility is obvious to anyone who sees the site.’

Kaiser (The Old Testament Documents: Are They Reliable And Relevant?) agrees that:

‘Since Jericho is located in the most famous Rift Valley where earthquakes are frequent, it should come as no surprise that the Jordan was dammed up or that Jericho’s collapse may have resulted from a serious aftershock.’

Hess concurs:

‘From a geological perspective, the Jordan River Valley lies at the juncture of tectonic plates that create an unstable region. Earthquakes can occur and have been known to block the flow of the river.’

But, Hess adds:

‘No mention of an earthquake appears in the account in Joshua. Whatever secondary causes there were, the primary purpose was the exaltation of Israel’s God and his people.’

Although it is of some interest to consider a possible mechanism for this event (so that it becomes a ‘miracle of timing’, rather than a divine act totally separated from the laws that God has built into nature) the text is specially interested in the theological implications.  These are well drawn out by Firth (BST), who notes

(a) the similarity between the crossing of the Reed Sea (marking the beginning of the wilderness wanderings) and the crossing of the Jordan (marking their end).

(b) that this miracle is a witness to the Israelites and all the surrounding nations that Yahweh is Lord of all.

Some practical implications:

1. God is in complete control of every aspect of his universe
2. God never fails
3. God’s timing is perfect
4. God’s plan for our lives can be trusted
5. God’s plan is the best plan for our lives

(Source)