Thou didst cleave the fountain and the flood - That is, the source of the streams and the streams themselves. The main allusion is probably to the Jordan, and the idea is, that God had, as it were, divided all the waters, or prevented any obstruction to his people from the river in any respect; 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.
Thou driedst up mighty rivers - Margin, "rivers of strength." The Hebrew - איתן 'êythân - (compare Deuteronomy 21:4; Amos 5:24; 1-Kings 8:2) - means rather perennial, constant, ever-flowing. The allusion is to rivers or streams that flow constantly, or that do not dry up. It was this which made the miracle so apparent. It could not be pretended that they had gone over the bed of a stream which was accustomed to be dry at certain seasons of the year. They passed over rivers that never dried up; and, therefore, it could have been only by miracle. The main allusion is undoubtedly to the passage of the Jordan.

Thou didst cleave the fountain - Thou didst cleave the rock in the wilderness, of which all the congregation drank.
Thou driedst up mighty rivers - Does not this refer to the cutting off the waters of the Jordan, so that the people passed over dryshod?

Thou didst cleave the fountain and the flood,.... That is, the rocks at Horeb and at Kadesh, from whence water flowed as out of a fountain, and became a flood, whereby the people of Israel were supplied with water in the wilderness, and also their beasts; and from this instance it may be concluded that God will not leave his people, nor suffer them to want, but will supply all their need while they are in the wilderness, and will open fountains and rivers for them, Isaiah 41:17 he himself is a fountain of living water; Christ is the fountain of gardens, and the Spirit and his grace a well of living water springing up unto everlasting life:
thou driedst up mighty rivers; the river of Jordan, called "mighty", as Kimchi says, because by its strength it overflowed all its banks and "rivers", and because other rivers flowed into it; this was dried up, or way was made through it, as on dry land, for the people of Israel to pass into Canaan, Joshua 3:14, the Targum is,
"thou hast dried up the fords and brooks of Hermon, and the fords of Jabbok and Jordan;''
see Numbers 21:14, and the Lord, that did this, is able to dry up, and will dry up, the river Euphrates, as is foretold, Revelation 16:12, that is, destroy the Turkish empire, and make way for the spread of the Gospel in the eastern parts of the world; to which reference is had in Isaiah 11:15.

cleave the fountain--that is, the rocks of Horeb and Kadesh; for fountains.
driedst up--Jordan, and, perhaps, Arnon and Jabbok (Numbers 21:14).

The flood - Thou didst by cleaving the rock, make a fountain and a stream to flow from it, for the refreshment of thy people in those dry deserts. Driedst - Jordan and the Red Sea; for the sea itself; yea, a greater sea than that, is called a river, Jonah 2:3, where the Hebrew word is the same which is here used. And the same title is expressly given to the sea, by Homer, and other ancient writers.