This is one of the more significant chapters in Joshua. Like I said in the introduction to this book, the crossing of the Jordan river is meant to mimic the crossing of the Red Sea, and there are a couple reasons why.
- We know from the prior chapter that there are passable fords in the Jordan (Josh 2:7), although since it is flooding season (v. 15), it is possible that the fords are not safe or easy for a large group of people to cross, while two spies might be able to cross more easily.
- It is an active demonstration of the LORD's support for Joshua (v. 7). By performing a miracle while Joshua is leader of the nation, it shows the nation that the LORD is affirming Joshua and will consolidate his leadership.
- The prior generation that saw the crossing of the Red Sea was entirely wiped out in the desert. Crossing the Jordan river will demonstrate to the current generation the miraculous powers of the LORD, giving them faith that they can be victorious over the Canaanite nations.
The last reason is probably the most significant, and in part it explains why the book of Joshua contains so many parallels to the Pentateuch. Because the prior generation was wiped out, the LORD is now redoing many of the things that he did with the last generation, because in both cases their objective was to invade the promised land. They scout the land for information and need miracles to demonstrate the LORD's support for them. The knowledge of the prior generation can be passed down to the present, but the experiences die with the people who had them, and that is a very significant fact. Following the LORD is not a knowledge-based endeavor; it also demands a present experience with the LORD, because it is by seeing the miracles and power of the LORD for yourself that you build confidence in his leadership and abilities. Simply being told of the Red Sea crossing is enough to melt the hearts of the Canaanites, but it's not enough to supply the Israelites with confidence in the LORD. The same is true for people who follow the LORD today.
Other than that, crossing the Jordan is symbolic of baptism, just like crossing the Red Sea. This consecrates the Israelites for their invasion of the promised land. This is also possibly why the water was stopped up at Adam, because it implicitly refers to the curse of Adam which has been wiped off the Israelites. I think that's everything I want to say about this chapter.