Albert Barnes Commentary Ecclesiastes 1:7

"All the rivers run into the sea, yet the sea is not full; unto the place whither the rivers go, thither they go again." — Ecclesiastes 1:7 (ASV)

The place - that is, the spring or river-head. It would seem that the ancient Hebrews regarded the clouds as the immediate feeders of the springs (Proverbs 8:28, and Psalms 104:10, Psalms 104:13). Genesis 2:6 indicates some acquaintance with the process and result of evaporation.