John Calvin Commentary Exodus 7:19

"And Jehovah said unto Moses, Say unto Aaron, Take thy rod, and stretch out thy hand over the waters of Egypt, over their rivers, over their streams, and over their pools, and over all their ponds of water, that they may become blood; and there shall be blood throughout all the land of Egypt, both in vessels of wood and in vessels of stone." — Exodus 7:19 (ASV)

And the Lord spoke to Moses. This is the more extended narrative of which I spoke. Moses mentions nothing different from what happened before but explains more clearly his mode of action in performing the miracle: namely, that what God had commanded was completed by the instrumentality of Aaron.

There was a reason for beginning with this miracle, so that the Egyptians might know that there was no safeguard for them in the resources upon which they prided themselves the most.

We know what great wealth, defense, and conveniences came to them from the Nile. From it came their abundant fisheries; from it came the fertility of their whole country, which it irrigated in its flooding—something that is harmful in other lands. Its navigation was most advantageous for their merchants, and it was also a strong fortification for a good part of the kingdom.

Therefore, to cast down the Egyptians from their principal dependence, He turns its waters into blood.

Besides, because water is one of the two elements of which human life consists, in depriving the Egyptians of one part of their life, He used the best and shortest method of humiliating their haughtiness, had they not been altogether intractable.

He might, indeed, by a single breath, have dried up all the sources of water and overwhelmed the whole nation with drought. However, this would have been commonly believed to have happened by chance, or naturally, and therefore would have been a less apparent miracle, while it would have prevented others.

It would, then, have been sufficient, by the terror of death it awakened, to turn them to the fear of God, unless their madness had been desperate.

Moses enumerates, besides the river, the streams, ponds, and pools of water. This is because, in different parts of the country, both artificially and naturally, the Nile was so diffused that scarcely any other country is provided in all directions with such an abundance of water. It is as though God were saying, “It will profit you nothing to possess such an immense supply of water, because you will thirst as much as if the Nile were dry.”

He adds, both in vessels of wood and in vessels of stone; meaning that in whatever kind of vessel they came to draw, they would find nothing but blood.