The short answer is God dried up the waters of the Jordan River for Israel in the same manner as He did with Moses and the Israelites crossing the Red Sea. The Israelites’ crossing of the Jordan River on dry land was of tremendous significance to the Israelites. Joshua explained the significance of this event before it took place,
“And Joshua said, Hereby ye shall know that the living God is among you, and that he will without fail drive out from before you the Canaanites, and the Hittites, and the Hivites, and the Perizzites, and the Girgashites, and the Amorites, and the Jebusites. Behold, the ark of the covenant of the Lord of all the earth passeth over before you into Jordan. Now therefore take you twelve men out of the tribes of Israel, out of every tribe a man. And it shall come to pass, as soon as the soles of the feet of the priests that bear the ark of the Lord, the Lord of all the earth, shall rest in the waters of Jordan, that the waters of Jordan shall be cut off from the waters that come down from above; and they shall stand upon an heap.” Joshua 3:10–13
The crossing affirmed God’s presence with them and His promise to remove their enemies from the land.
“That this may be a sign among you, that when your children ask their fathers in time to come, saying, What mean ye by these stones? Then ye shall answer them, That the waters of Jordan were cut off before the ark of the covenant of the Lord; when it passed over Jordan, the waters of Jordan were cut off: and these stones shall be for a memorial unto the children of Israel for ever.” Joshua 4:6–7
The memorial was to serve as a lasting sign of God’s work among the Israelites.
The conclusion of this event offers an additional insight into its significance and the reason for the memorial.
“For the Lord your God dried up the waters of Jordan from before you, until ye were passed over, as the Lord your God did to the Red sea, which he dried up from before us, until we were gone over: That all the people of the earth might know the hand of the Lord, that it is mighty: that ye might fear the Lord your God for ever.” Joshua 4:23–24
First, a comparison is made with the crossing that took place at the Red Sea under the leadership of Moses. This emphasis on God’s provision was an ongoing sign to the people of Israel.
Second, there was an emphasis on the power of God. The miracle was done “so that all the peoples of the earth might know that the hand of the Lord is powerful.” No other god could compare in power. The gods of Israel’s enemies were created things that had no ability to move water and provide dry passage across a river.
Third, the result of the miracle was that the Israelites would fear the Lord forever. The idea was that this miracle would leave the people in such awe that they and their descendants would talk about it and live in the fear of God and worship Him as a result.
God showed His power and presence in the crossing of the Jordan, and the memorial set up by His people served as a reminder of His might and why the Israelites should fear the Lord. Both the act and its memorial worked to point to God’s glory, presence, and strength, which would empower the Israelites as they took possession of their land in the days ahead.