Albert Barnes 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)

And the third angel poured out his vial upon the rivers and fountains of waters. This coincides also with the account of the sounding of the third trumpet (Revelation 8:10–11): And the third angel sounded, and there fell a great star from heaven burning as a lamp, and it fell upon the third part of the rivers, and upon the fountains of waters. As to the meaning of the phrase, "rivers and fountains of waters," see Barnes on Revelation 8:10-11.

We found, it was supposed, in the application of that passage, that the invasion of the Roman empire by Attila, king of the Huns, was referred to, affecting mainly those parts of the empire where the rivers and streams had their origin. The analogy would lead us, in the fulfillment of the passage before us, to look for some similar desolations on those portions of Europe. See Barnes on Revelation 16:7.

And they became blood. This would properly mean that they became as blood, or became red with blood; and it would be fulfilled if bloody battles were fought near them so that they seemed to run blood.