‘Was YHWH displeased against the rivers?
Was your anger against the rivers,
Or your wrath against the sea?
That you rode upon your horses,
On your chariots of salvation?
The opening up of the River Jordan and of the Reed Sea (Yam Suph) may be in mind here, but with wider reference to the rivers of the world. The point being that He smote them in order to go forth on His heavenly horses and chariots of deliverance (2 Kings 6:17), with and on behalf of His people.
It was not that He was angry with the rivers or the seas, but because He was angry with those whom they represented. The nations of the world were often denoted in connection with their rivers. See Isaiah 27:1 (which is probably speaking of the Tigris, the Euphrates, and the Nile); Ezekiel 29:3; Isaiah 7:18; Isaiah 8:7, and the sea often denotes peoples (Psalms 65:7; Daniel 7:3; Isaiah 23:11). But YHWH's anger is against the peoples, not against river and sea. It was against the peoples that He rode with His heavenly host, (and still rides on behalf of His people), on horses, and on His chariots of deliverance, as He will also at the end of time (Revelation 19:11-16).
YHWH smote the Jordan as He led His people victoriously into the land of Canaan. But it was not because of His anger against the Jordan, but because of His anger against the people beyond the Jordan for whom the time of their judgment had come. Zechariah pictures the destruction of nations in judgment as connected with the smiting of the rivers and the sea (Zechariah 10:11), while the Psalmists speak of the smiting of river and sea as denoting the powerful activity of God (Psalms 89:25; Psalms 114:3; Psalms 78:13) mainly because of the smiting of the Reed Sea and the Jordan.
So the whole picture that is being presented is that of God's great deliverance of His people, both as something that has happened, and as something that, in the mind of the prophet, continually happens. Wherever God's true people are, there are the horsemen and chariots of YHWH (compare 2 Kings 6:17).