Exodus 7:17
Thus saith the LORD, In this thou shalt know that I am the LORD: behold, I will smite with the rod that is in mine hand upon the waters which are in the river, and they shall be turned to blood. Thus saith the LORD, In this thou shalt know that I am the LORD: behold, I will smite with the rod that is in mine hand upon the waters which are in the river, and they shall be turned to blood.
Thus saith the LORD, In this thou shalt know that I am the LORD, was the advance announcement of the first plague as the sign by which Pharaoh would know that the LORD is God, that at God's command the waters of the river would be struck with the rod and become blood. The warning described the plague and its effects beforehand so that Pharaoh could obey the command to free Israel and avoid the judgment, but he refused and hardened his heart.
The plague fell upon the Nile because the Egyptians worshiped the river as a god and gave it the devotion belonging to God alone. They credited it with sustaining life and crops, with quenching thirst, with watering the land to make crops grow, and with supplying fish for food. They treated the Nile as sacred, revered its fish, and believed its water had divine power to heal. Pharaoh went to the river daily, standing on its banks each morning to pray and to offer praise and thanksgiving to the water for its great good and power, saying that without it they could not live.
In this setting the turning of the water to blood was God's judgment upon one of Pharaoh's exalted gods and upon idolatrous trust. It demonstrated that the LORD, not the river, held power over it, that Jehovah is the true God, and that Egypt's revered powers cannot protect their worshipers. It was God's miraculous interposition on behalf of Israel and a calamity visited upon the Egyptians for persecuting His chosen people.
At the word of the LORD, Moses, with Aaron, struck the waters before Pharaoh and his great men, and the river and the water kept in household vessels and cisterns became blood. All the fish died, and the river became a foul, putrid, stinking mass that the Egyptians loathed to drink. The people could not use the water for any purpose, and the condition continued for seven days. The plague was proof that the LORD had sent Moses and Aaron and evidence of God's power and judgment.
The Egyptian magicians imitated the sign, which led Pharaoh to dismiss the miracle, joined with the rod becoming a serpent, as performed by magic. He remained unmoved, refused to yield, and hardened his heart through the seven days without changing his response. Instead of softening him, the judgment increased his hatred of the Israelites. Remembering this deliverance should afterward have kept Israel trusting in God's protecting power and recognizing His hand holding back surrounding evils when walled in at the Red Sea, instead of reproaching Moses, fearing death by hunger and fatigue or by war, and wishing they had remained in bondage.