8 And it shall come to pass, if they will not believe thee, neither hearken to the voice of the first sign, that they will believe the voice of the latter sign. 9 And it shall come to pass, if they will not believe also these two signs, neither hearken unto thy voice, that thou shalt take of the water of the river, and pour it upon the dry land: and the water which thou takest out of the river shall become blood upon the dry land.
After turning the staff into a serpent, the healthy hand leprous, and then changing them back, God tells Moses how to perform another miracle. This one is different from the other two, though. For one thing, God does not actually rehearse this one with Moses, telling him only to do it in the land of Egypt. For another, while there is still a transformation of something pure into something unnerving, there is no return transformation back to the original.
It seems to me that Moses didn’t act out this last wonder by himself because its message was directed specifically to the people of Israel and Egypt. It was not intended to test him personally as the first two miracles had been.
So, what was the special message that it conveyed to the people in Egypt? It seems clear to me that it was a reminder of evil done and a promise for retribution. Recall the former Pharaoh’s instructions that “every son that is born [of the Israelites] ye shall cast into the river.” Now Moses was to go to that same river, draw from it, and pour blood upon the land. It was the blood of the innocent, the blood of the infant sons killed by Pharaoh’s decree. God had not been blind to this horrific injustice. He had collected the full measure of blood, and now He would pour the same upon the people of Egypt. As it would be written, “vengeance is mine, saith the Lord” (Romans 12:19). This is why this sign would not only be shown to the assembly of Israelites, but also to Pharaoh himself. Both the ruler and the Israelites needed to understand that God remembered what was done, and He had come to repay!