Jos 3:9-17 And Joshua said to the people of Israel, “Come here and listen to the words of the LORD your God.” And Joshua said, “Here is how you shall know that the living God is among you and that he will without fail drive out from before you the Canaanites, the Hittites, the Hivites, the Perizzites, the Girgashites, the Amorites, and the Jebusites. Behold, the ark of the covenant of the Lord of all the earth is passing over before you into the Jordan. Now therefore take twelve men from the tribes of Israel, from each tribe a man. And when the soles of the feet of the priests bearing the ark of the LORD, the Lord of all the earth, shall rest in the waters of the Jordan, the waters of the Jordan shall be cut off from flowing, and the waters coming down from above shall stand in one heap.” So when the people set out from their tents to pass over the Jordan with the priests bearing the ark of the covenant before the people, and as soon as those bearing the ark had come as far as the Jordan, and the feet of the priests bearing the ark were dipped in the brink of the water (now the Jordan overflows all its banks throughout the time of harvest), the waters coming down from above stood and rose up in a heap very far away, at Adam, the city that is beside Zarethan, and those flowing down toward the Sea of the Arabah, the Salt Sea, were completely cut off. And the people passed over opposite Jericho. Now the priests bearing the ark of the covenant of the LORD stood firmly on dry ground in the midst of the Jordan, and all Israel was passing over on dry ground until all the nation finished passing over the Jordan.
“God extends the force of the miracle beyond their entrance into the land, and properly so, since the mere opening of a way into a hostile country from which there would be no retreat, would be nothing but exposure to death. For they would either easily fall, through being entangled in difficulties and in an unknown region, or they would perish through want. Joshua therefore foretold, that when God drove back the river it would be as if He had stretched out His hand to strike all the inhabitants of the land, and that the proof which He gave of His power in their crossing the Jordan would be a certain presage of victory, to be gained over all the tribes.” (Keil)
God had apparently revealed to Joshua how the Jordan would become a dry bed and passable by Israel. The waters would not be divided, as they were at the Red Sea (Exodus 14:21-22). Instead, the waters of the Jordan would be cut off upstream, leaving a dry riverbed before Israel. This was not the time of year when the Jordan was reduced to a trickle. Because of the spring rains, the time of early harvest, the river was swollen and overflowing its banks.
“In some respects the passage of the Jordan was more strikingly miraculous than that even of the Red Sea. In the latter God was pleased to employ an agent; the sea went back by a strong east wind all that night, and made the sea dry land, Exodus 14:21. Nothing of this kind appeared in the passage of the Jordan; a very rapid river (for so all travellers allow it to be) went back to its source without any kind of agency but the invisible hand of the invisible God.” As well, even with the flow of the river stopped, it was miraculous that the people could cross over on dry ground. God miraculously dried the riverbed so that they didn’t slog through marshy mud.
In some sense, it took greater faith for Israel to cross the Jordan River than it took for them to cross the Red Sea. At the Red Sea, Israel was pursued by the Egyptian army (Exodus 14:8-28). Crossing the Red Sea was to travel away from danger, and to put a barrier between Israel and the danger pursuing them. In crossing the Jordan River, Israel travelled towards potential danger, the many Canaanites who would war against them. When Israel crossed the Jordan River, they cut off their path of retreat and could be “trapped” and slaughtered in Canaan. Crossing the Jordan River mean that Israel was completely committed to the task of conquering the land of Canaan; they were left with no other option. This was a demonstration of great faith. (Guzik)