You are imagining the water source as a river and that's not my idea. My idea of the water source is more like a cup of wine with poison in it, thus no incoming flow. The only flow comes from the already tainted shard, antimattered creature, vampire creature, or feral bond, etc. There's no new flow replacing the tainted flow in the shard, antimattered creature, vampire creature, or feral bond, etc. This spell was intended to represent a potent venom that could kill a person in only one use, but because this is a game it would be no fun if it killed the opponent in 1 turn, therefore it does damage over time when infecting something.
Obviously if this card requires to be a permanent to be balanced then it could be changed to that, but right now I don't think this card is OP in any way except for cost.