Albert Barnes Commentary Psalms 104:10

"He sendeth forth springs into the valleys; They run among the mountains;" — Psalms 104:10 (ASV)

He sendeth the springs into the valleys - Though the waters are gathered together into seas, yet God has taken care that the earth shall not be dry, parched, and barren. He has made provision for watering it; and by a most wise, wonderful, and benevolent arrangement, He has formed springs among the valleys and the hills. It is now living nature that comes before the psalmist's eye, and all this he traces to the fact that the earth is “watered” and is not a waste of rocks and sands.

The allusion in this part of the psalm (see the Introduction) is to the earth as covered with vegetation—or, to the third day of the week of creation (Genesis 1:9–13), which, in Genesis, is connected with the gathering of the waters into seas. This description continues in Psalm 104:18. The literal rendering here would be, sending springs into the valleys. He conducts the waters from the great reservoirs—lakes and seas—in such a way that they form springs in the valleys.

The way in which this is done is among the most wonderful and benevolent in nature. This occurs by that power, derived from heat, by which the waters of the ocean—contrary to the natural law of gravitation—are lifted up in small particles, as vapor. These particles are then carried by the clouds where they are needed and fall upon the earth to water the plants and to form fountains, rivulets, and streams. They are thus carried to the highest mountains to be filtered through the ground, forming springs and streams below.

Which run among the hills - The margin says, “walk.” That is, they go between the hills. The streams of water flow along in the natural valleys that have been made for them.