You've got to have the full picture before you can understand what Israel was looking at. I was in So. Cal. getting ready to head to Oakland for mission work. We were staying at a parsonage in a church. I had an old van with cracked ring (it smoked badly) and nearly bald tires. The people of the church said don't go. I realized the loved us and were concerned. I began to doubt that it was really the Lord that told us to go.
I went for a walk to pray and meditate, as I had a wife and two kids to go with us. The Lord really started to talk to me to assure me I was on course. He told me to go to the parsonage and lookup Joshua chapter 3.
When I got there I started reading. The Lord command the people stay back 2000 cubits. He had the Levites that carried the ark head for the Jordan. The Lord said imagine yourself standing there with Israel watching this happen. Scripture says them were up to their soles in the river. He asked me what are you thinking as the river was in flood stage. I said I think that is the last we will see of these Levites and our ark. God said that he wanted it to look bad so they would have to use their faith to go on.
Needless to say, I was ready to head North being assured that God was with me. We didn't even know anyone in Oakland. It appeared we would not quite have enough gas money to get there, but we left anyway. We had pre-arranged to stop at my cousins house in Modesto to visit. That cousin slipped me a twenty dollar bill for the Lord's work. It gave us just enough gas money.
The answer to the question is God drew the waters back into a heap. Interesting also is they walked on dry ground in the riverbed. Every river I have been in had a muddy bottom.