They go up by the mountains - That is, when they were gathered together into seas. They seemed to roll and tumble over hills and mountains, and to run down in valleys, until they found the deep hollows which had been formed for seas, and where they were permanently collected together. The margin here is, "The mountains ascend, the valleys descend." So it is translated in the Septuagint, in the Latin Vulgate, by Luther, and by DeWette. The more natural idea, however, is that in our translation: "They (the waters) go up mountains; they descend valleys."
Unto the place - The deep hollows of the earth, which seem to have been scooped out to make a place for them.
Which thou hast founded for them - Where thou hast laid a permanent foundation for them on which to rest; that is, which thou hast prepared for them.

They go up by the mountains; they go down by the valleys - Taking the words as they stand here, springs seem to be what are intended. But it is difficult to conceive how the water could ascend, through the fissures of mountains, to their tops, and then come down their sides so as to form rivulets to water the valleys. Most probably all the springs in mountains and hills are formed from waters which fall on their tops in the form of rain, or from clouds that, passing over them, are arrested, and precipitate their contents, which, sinking down, are stopped by some solid strata, till, forcing their way at some aperture at their sides, they form springs and fountains. Possibly, however, vapours and exhalations are understood; these by evaporation ascend to the tops of mountains, where they are condensed and precipitated. Thus the vapours ascend, and then come down to the valleys, forming fountains and rivulets in those places which the providence of God has allotted them; that is, continuous valleys, with such a degree of inclination as determines their waters to run in that direction till they reach another river, or fall into the ocean.
Some have thought there is a reference to the breaking up on the fountains of the great deep, at the time of the flood; while the protrusion of the waters would raise the circumambient crust, so as to form mountains, the other parts, falling in to fill up the vacuum occasioned by the waters which were thrown up from the central abyss, would constitute valleys.
Ovid seems to paraphrase this verse: -
Jussit et extendi campos, subsidere valles,
Fronde tegi sylvas, lapidosos surgere montes.
Met. lib. i., ver. 43.
"He shades the woods, the valleys he restrains
With rocky mountains, and extends the plains."
Dryden.

They go up by the mountains, they go down by the valleys,.... The Targum is,
"they ascend out of the deep to the mountains;''
that is, the waters, when they went off the earth at the divine orders, steered their course up the mountains, and then went down by the valleys to the place appointed for them; they went over hills and dales, nothing could stop them or retard their course till they came to their proper place; which is another instance of the almighty power of the Son of God. Some render the words, "the mountains ascended, the valleys descended (m)"; and then the meaning is, when the depth of waters were called off the earth, the mountains and valleys appeared, the one seemed to rise up and the other to go down; but the former reading seems best, and emblematically describes the state of God's people in this world, in their passage to their appointed place; who have sometimes mountains of difficulties to go over, and which seem insuperable, and yet they surmount them; sometimes they are upon the mount of heaven by contemplation, and have their hearts and affections above; they mount up with wings as eagles; sometimes they are upon the mount of communion with God, and by his favour their mount stands strong, and they think they shall never be moved; at other times they are down in the valleys, in a low estate and condition; in low frames of soul, in a low exercise of grace, and in the valley of the shadow of death, of afflictive providences in soul or body: and as the waters, thus steering their course under a divine direction, and by an almighty power, at length came unto the place which, the psalmist says, thou hast founded for them, meaning the seas; which the Lord founded and prepared for the reception of them; and which collection of waters in one place he called by that name, Genesis 1:10. So the Lord's people, through a variety of circumstances, trials, and exercises, will be all brought safe to the place appointed for them, and prepared by Christ in his Father's house; where they will be swallowed up in the boundless ocean of everlasting love.
(m) So Pagninus, Musculus, Cocceius.

Psalm 104:8 continues with the words אל־מקום (cf. Genesis 1:9, אחד אל־מקום): the waters retreat to the place which (זה, cf. Psalm 104:26, for אשׁר, Genesis 39:20) God has assigned to them as that which should contain them. He hath set a bound (גּבוּל, synon. חק, Proverbs 8:29; Jeremiah 5:22) for them beyond which they may not flow forth again to cover the earth, as the primordial waters of chaos have done.

Go up - In that first division of the waters from the earth, part went upwards, and became springs in the mountains, the greatest part went downwards to the channels made for them.