Exodus 7:20
And Moses and Aaron did so, as the LORD commanded; and he lifted up the rod, and smote the waters that were in the river, in the sight of Pharaoh, and in the sight of his servants; and all the waters that were in the river were turned to blood.
At God's command Moses and Aaron acted in exact obedience, lifting the rod and smiting the waters in the river in the sight of Pharaoh and his servants and officials and great men, and all the waters that were in the river were turned to blood. The stroke on the river counts as the second miracle wrought against Egypt. Before bringing this plague, as with each plague, Pharaoh was faithfully told beforehand its nature and its effects, so that he would have the privilege of saving himself from the judgment by letting Israel go to worship and sacrifice to God to serve Him, with further judgments to follow only if he refused to obey God's command.
The plague fell on the waters because the Nile was worshiped as a god and treated as the source of Egyptian existence, food, and wealth. The Egyptians depended on it to quench thirst, to water their lands to make vegetation flourish, and to supply fish for their tables, they revered its water and its fish and believed its waters had divine power to heal, and Pharaoh went to its banks every morning to offer praise and thanksgiving to the water for its great good and power. The judgment turned water stored in houses and in household cisterns to blood as well as the river, killed the fish, made the river foul and a putrid mass, and left the people unable to use the water for any purpose, so that they had to dig wells. The plague continued for seven days.
By turning the adored waters to blood and making the fish putrid, the verse exposes the river's impotence, strikes at Egyptian idolatry, and shows that its life-giving power belongs to God. The turning of the water to blood was a miracle wrought through Moses by God's power, not by any human influence or power in Moses himself. It confirmed that God had sent Moses and Aaron, was done to convince Pharaoh that the great I AM sent Moses to command him to let Israel go that they might serve Him, and was God's striking of Egypt with calamity for persecuting His chosen people as a miraculous manifestation of His power in Israel's favor and proof of His protecting power and His hand holding back the evils around them. Moses' work in breaking the yoke of bondage prefigured Christ's first advent to break Satan's power over the human family and deliver those made captives by him.
Satan worked through Pharaoh's magicians by their enchantments to counterfeit this true work of God, to destroy its influence, to harden Pharaoh's heart against God's power, to stagger the faith of Moses and Aaron in their divine mission, to keep Israel from being released to serve God, and to destroy faith through all ages in Christ's mighty miracles. Yet God's work remained superior to Satan's, for the magicians could imitate only a few of the miracles and God at last cut off the channel through which Satan worked. Pharaoh despised their words and would not listen, yet he had no power to harm them; he did not humble himself but hardened his heart, his resolve was unchanged for the seven days, and he refused to yield. The sign, like the rod's becoming a serpent, did not soften him but increased his hatred of the Israelites.
Pharaoh believed the miracle was performed by magic because of his magicians' work. Full evidence that it was not magic but a judgment from the God of Heaven came only later when the plague of frogs was removed and the dead frogs were left in heaps before the Egyptians corrupting the atmosphere, which their vain philosophy could not explain away. The proof of God's power in turning the Nile to blood by the rod in Moses's hand should have kept Israel trusting God when walled in at the Red Sea, but in frantic terror they forgot it along with all God's other miraculous interpositions for them, reproached Moses, and expected death instead of deliverance.