Joshua 3

3 The ark represented the presence of God. We too should follow God wherever He leads us to go. The next verse tells us that the people would not know where to go, since they had not been here before. God always knows the best approach to any problem, so it is best to follow Him.
4 God's holiness demanded that the people (except the priests carrying the ark) remain about a kilometer (or about 0.6 miles) behind.
5 As a people on God's mission, they were to set themselves apart (i.e., sanctify themselves). They were to make a special effort to seek God and be obedient to Him in their daily lives.

God calls us to be sanctified to Him every day, but there are special occasions when it is appropriate to bring sanctification to our immediate attention. On these occasions, we re-commit ourselves to God, especially if we have become negligent in our relationship with or obedience to Him.

7 God does several miracles for Joshua that parallels those He performed while Moses was the leader. The purpose was to rally the people behind Joshua's leadership as Joshua sought leadership from God. There was no descension against Joshua at this time, but God wanted to make it clear that Joshua was His chosen leader so that the people would not rebel against his commands during this crucial stage in their history.
8 Joshua was to instruct the priests before he addressed the rest of the people (they are already following his instructions in Josh 3:11).
9 Joshua does not indicate here that one of the purposes of the eminent miracles was to confirm his leadership. Instead, he focuses on it as a confirmation that God was continuing to fulfill His promise to Abraham (Gen 15:18-21).
10 During the incident recorded in Gen 9:22-27, Noah cursed his grandson, Canaan. All the tribes listed here, except for the Perizzites, were his descendants (Gen 10:15-17). Israel was descended from Shem and as prophesied in Noah's curse they would now drive out and enslave the descendants of Canaan. We do not know where the Perizzites came from, but they are always linked with the Canaanites, and undoubtedly adopted their ungodly culture and therefore shared in their judgement.
11 The pillar of cloud and fire is no longer mentioned. The ark would now serve as the reminder of God's leadership and presence as Israel invaded and remained in Canaan. As Joshua spoke, the priests were carrying the ark towards the Jordan River.
13 Joshua predicts the upcoming miracle so that the people will believe that God truly did it. The prediction, timing, and the resulting glorification of God make this an indisputable miracle, and not just a coincidence.
14 In Josh 3:4, the Israelites had been instructed to stay approximately 1 kilometer (0.6 miles) from the ark. Only those assigned to gather stones from the riverbed would be allowed to approach it. The rest would cross further downstream.
15 The enormity of the miracle is increased because the river was running at flood stage. While the river is not wide during most of the year, it can be up to a mile wide at places during flood stage (March-April). It is unlikely that the people would have been able to cross the rapidly flowing river at this time except for God's intervention. There would not have been a ford large enough to accommodate approximately 1.5 million people (nine-and-a-half tribes plus the army from the others).
16 Some commentators would dismiss this miracle and suggest that a landslide blocked the river for a time. This has, in fact happened in recorded history -- 1267, 1906, and 1927. However, the description here indicates that the water stood up in a heap. There was not a physical barrier -- God was holding up the water with His power. Notice that in the parting of the Red Sea that the water stood up on both sides, indicating that the Sea was a deep and standing body of water. In a river situation, the flow only needed to be stopped upstream. The Jordan ran dry from the town of Adam to the Dead Sea while the Israelites crossed. Israel crossed about sixteen miles downstream of Adam, across from Jericho, but the entire 65 miles from Adam to the Dead Sea were dried up.
17 The ground was also dry, which indicates that this wasn't a simple stoppage of water. God dried out the riverbed so that the Israelites would not have to contend with mud and puddles of water as they hurried across (Josh 4:22,23).