Revelation 16:4 - Clarke's commentary and critical notes on the Bible

And the third angel poured out his vial upon the rivers and fountains of waters; and they became blood. Upon the rivers and fountains of waters - This is an allusion to the first Egyptian plague, Exodus 7:20; and to those plagues in general there are allusions throughout this chapter. It is a sentiment of the rabbins that "whatever plagues God inflicted on the Egyptians in former times, he will inflict on the enemies of his people in all later times." See a long quotation on this subject from Rabbi Tanchum in Schoettgen.