As Creator, the Lord controls the waters. He rebukes the sea, which is often a symbol of chaos and power in the Old Testament. The reader is probably also being reminded of the parting of the Red Sea to save Israel and to then destroy the Egyptian army during the Exodus.
The Canaanites often described Baal as demonstrating his authority by his defeat of the sea. Nahum is showing that the Lord is the One who truly rules over the waters. He can dry up rivers through drought as an act of judgment.
When Jesus calms the sea (Luke 8:22–24) he shows that he is ultimately the Lord of whom Nahum speaks.1
Bashan, Carmel, and Lebanon describe the northern parts of Israel, and they represent the most fertile parts of the land. The Lord who controls the water can even cause these fertile lands to dry up and wither. If the Lord has judged Israel in the past in this way, how much more severe will his judgment of Nineveh be?2 Throughout the Scriptures there is a connection between the coming of God as judge and the withering of nature (Isaiah 24:1–13).3
4 He rebukes the sea and makes it dry; he dries up all the rivers; Bashan and Carmel wither; the bloom of Lebanon withers.