Albert Barnes Commentary Psalms 74:15

"Thou didst cleave fountain and flood: Thou driedst up mighty rivers." — Psalms 74:15 (ASV)

You cleaved the fountain and the flood – that is, the source of the streams and the streams themselves.

The main allusion is probably to the Jordan. The idea is that God had, so to speak, divided all the waters, or prevented the river from obstructing his people in any way; it was as if the waters in the very springs and fountains, and the waters in the channel of the river flowing from those springs and fountains, had been so restrained and divided that there was a safe passage through them (Joshua 3:14–17).

You dried up mighty rivers – Margin: “rivers of strength.” The Hebrew term, איתן ('êythân) – compare Deuteronomy 21:4; Amos 5:24; 1 Kings 8:2 – means, more accurately, perennial, constant, ever-flowing.

The allusion is to rivers or streams that flow constantly, or that do not dry up. This is what made the miracle so apparent.

It could not be pretended that they had crossed the bed of a stream that was accustomed to being dry at certain seasons of the year. They crossed rivers that never dried up; and, therefore, it could only have been by miracle. The main allusion is undoubtedly to the passage of the Jordan.