A.T. Robertson Commentary Revelation 16:4

"And the third poured out his bowl into the rivers and the fountains of the waters; and it became blood." — Revelation 16:4 (ASV)

Into the rivers and the fountains of waters (εις τους ποταμους κα τας πηγας των υδατων). See 8:10 for this phrase. Contamination of the fresh-water supply by blood follows that of the sea. Complete again.