He opened the rock - See the notes at Psalm 78:15. "They ran in the dry places like a river." Or, "a river." They flowed along in the desert - a river of waters. See the notes at 1-Corinthians 10:4.

He opened the rock, and the waters rushed out - See the note on Exodus 17:6, to which I can now add, that a piece of this rock, broken off by the hand of my nephew, E. S. A. Clarke, in the course of the present year [1822,] now lies before me. It is fine granite; and so well distinguished as a granite, that the feldt-spar, the mica, and the quartz, of which granite is composed, appear very distinctly. It is worthy of remark, that, as granite is supposed, in the most accredited systems of geology, to be the very basis of the earth, the original rock, and all other substances to be superimpositions upon it, and as the decompositions of the feldt-spar produce pure vegetable earth, this rock should be used for this purpose, and should be an emblem of Jesus Christ, the Creator and Redeemer of the human race; and that it should signify him who is the basis of all things; who upholds all by the word of his power; without whom nothing is stable, nothing fruitful; from whom alone the water of life proceeds; and in whose name only is salvation. And that rock (in the wilderness) was Christ! and it is the only remaining emblem of him in creation.

He opened the rock, and the waters gushed out,.... That is, God opened the rock, who before is said to spread the cloud, and bring the quails, for it was a miraculous affair; or Moses, by divine orders, which was done by smiting it, when waters flowed out in great abundance, sufficient to give drink to men and beasts, and which continued, Exodus 17:6. This was typical of Christ the Rock, 1-Corinthians 10:4, and of the opening of his side, from whence flowed blood and water, John 19:34 and of his being smitten with the rod of justice, and by the law of Moses, from whom flow abundance of grace, and the blessings of it, freely to all his people; See Gill on Psalm 78:15. See Gill on Psalm 78:16.
They ran in the dry places like a river; and did not sink and soak into them, but continued their flow, and followed the Israelites, wherever they went; see 1-Corinthians 10:4.

River - They flowed in channels which God provided for them, and followed the Israelites in their march.