‘And the third poured out his bowl into the rivers and the fountains of the waters, and it became blood.'
Now it is man's water supply that is attacked in some way. The same applies as on v. 3. Here however there is probably a reference to bloody warfare, as suggested by Ezekiel 32:6 where the blood of the armies of Egypt fill the waters. John's world was familiar with such warfare and it is a characteristic of history. But here we learn it is among other things the sign of God's wrath against sin. In this modern day we could see biological and chemical warfare affecting man's drinking supply.
The phrase ‘rivers and fountains of the waters' is repeated from Revelation 8:10. There is clearly a parallel activity in mind. These blessings, which were intended to provide man with lifegiving water and fruitfulness for his fields, will instead become a curse. In both cases there is possibly a deliberate contrast with the life-giving waters of Revelation 7:17 and Revelation 22:1. For the unbeliever there will be no soul refreshment.