What is the similarly to Moshe?
Radak: Just like I split the sea in front of Moshe, so I will split the Yarden in front of you, and through this [all Yisrael] will know that just like I was with Moshe, so I will be with you.
How will the miracle show the greatness of Yehoshua? Perhaps it is in the merit of Yisrael?
Malbim #1: Yisrael will turn back, and let the Aron and Yehoshua 1 go in front, and then the river will split, and his merit will be known.
Malbim #2: The miracle was not needed to cross the Yarden. It is not such a great river. Yakov crossed it with his staff, and the spies crossed it without a miracle! 2
People could think that the miracle is due to the Aron alone! (PF)
Perhaps the miracle was needed to enable Bnei Yisrael cross with all their animals! Everyone left Egypt with at least 90 donkeys of gold, silver and clothing (Bechoros, 5b). Even if Bnei Gad and Bnei Reuven had the biggest flocks, surely all the Shevatim had flocks! (PF)