The third vial is poured out on the rivers and the fountains and they become blood.  

Rivers and fountains symbolize the personal inner life, the human soul, the astral body. The source of life dries up for the people which strive to increase their own fame and power under the sign of the beast. For they have shed the blood of the saints and the righteous and you have given them blood to drink, for they are worthy. They have to drink their own blood.

Rivers in the landscape as symbols of the astral body

Schult (p.268) explains the picture as follows. The landscape is veined and vivified by water resources and fountains. They are a symbol for the desires in the soul, the astral body and the astral world. When the clear waters are transformed into red blood, the blood bound passions kill all higher aspirations. Lust makes that all spiritual forces die. Man falls from desire into lust and in the end to preparedness to kill saints. In this way karma is created and sooner or later the murdering will be revenged. The only alternative is purification of the inner desires. This is summoned from the altar in the holy part of the temple. And the angel calls to the judging God: Righteous you are,  which is and which was, you Lord, because you have judged this way. The addition ‘and which is to come’ is left out in the description of God, as after the sounding of the seventh trumpet, the future has become present.

The pouring out of the first three vials, Bamberg Apocalypse, 1000-1020.

Another example of intensification of the impacts

On the third rai of the spiral we see again the intensification of impacts. The opening of the third seal made the black horse appear and the horseman with a balance in his hand. What starts as trading and doing business leads to the desire for possessions at the cost of others. When the third trumpet blows we see the consequences of achieving one’s desires at any price in the star falling from heaven on one third of the rivers and water fountains which makes them bitter and causing those which drink of it die. And the third vial of wrath leads to the death of everybody following this path, as they have to drink their own blood.