From Chabad.org: The Jews journey on, making their way towards the eastern bank of the Jordan River. Encrypted in this Aliya is a great miracle which occurred when the Jews passed through the Arnon valley. Tall cliffs rose from both sides of this narrow valley, and in the clefts of these cliffs the Emorites, armed with arrows and rocks, were waiting to ambush the Jews. Miraculously, the mountains moved towards each other, crushing the Emorite guerrilla forces. This section ends with a song of praise for the well which sustained the Jews throughout their desert stay — and whose now-bloodied waters made the Jews aware of the great miracle which G‑d wrought on their behalf.
Look how far the Jews have come! Earlier in their tenure in the desert, they complained despite overt miracles, and now they sing the praises of G-d for a miracle they did not even witness (all they saw was their enemies’ blood in the streams). Either they all learned from their experiences, or the new generation had a different attitude about what they were experiencing. Either way, they are closer to being ready to enter Israel because of it!