New Defender's Study Bible Notes
7:17 turned to blood. This may not imply literal blood but rather “turned to appear like blood” Similar language is used in Joel 2:31, which speaks prophetically of God turning “the moon into blood.” See also II Kings 3:22, which says “the Moabites saw the water...as red as blood.” Whatever chemical or biological change took place, it made the water undrinkable and lethal even to the fish in the rivers. During the seven days that the plague persisted, the people had to dig for groundwater, from which the lethal contents of the water had apparently been filtered out (Exodus 7:24-25). One effect of this plague surely was that of undermining the Egyptian’s faith in their various river deities, especially Hapi, who personified the Nile.