Exodus 7:21
And the fish that was in the river died; and the river stank, and the Egyptians could not drink of the water of the river; and there was blood throughout all the land of Egypt. And the fish that was in the river died; and the river stank, and the Egyptians could not drink of the water of the river; and there was blood throughout all the land of Egypt.
The stroke upon the Nile was the first plague, turning its water to blood so that the fish died and the river stank and became foul, putrid and undrinkable, with blood throughout all the land of Egypt even in water stored in homes. The condition lay upon the waters for seven days, during which the people could use the water for no purpose and were forced to dig wells for drink.
The blow fell upon the Nile because Egypt depended upon it for drinking water, irrigation, food, wealth, vegetation and fish, and because the Egyptians worshiped the river and its fish as gods with devotion belonging alone to God. Pharaoh himself went to the river every morning to stand on its banks with praise and thanksgiving, recounting the great good it did and declaring its great power, saying that without it they could not exist. The plague thus turned a cherished object of idolatrous homage into a source of pollution, exposing the inability of the Nile and its fish to help or save themselves.
Moses and Aaron faithfully told Pharaoh beforehand the nature and effect of the plague, that he might know that the LORD sent them and know that the LORD is the LORD, and that he might have the privilege of saving himself from it by letting the people go to serve and sacrifice to God in the wilderness. Moses then smote the waters in the sight of Pharaoh and his officials, servants and great men, so that they saw the waters they adored turned to blood, and though Pharaoh despised the warning words and would not listen, he had no power to harm Moses and Aaron. The miracle was wrought by the power of God through Moses, not by any human influence or power, to convince Pharaoh that the great I AM sent Moses to command Pharaoh to let Israel go that they might serve Him.
In this work Moses prefigured Christ's first advent to break Satan's power over the human family and deliver those made captives by his power. Satan was upon the ground and worked through the magicians with their enchantments to counterfeit the true work of God for a two-fold purpose, to destroy the influence of what God wrought through Moses and to reach down through all ages to destroy true faith in the mighty miracles and works Christ would perform when He came, because Christ's coming would subject the power Satan held over mankind. His working through the magicians was calculated to harden Pharaoh's heart against God's power, to stagger the faith of Moses and Aaron in the divine origin of their mission, and to keep Israel from being released from Egyptian servitude to serve God. Yet God's work was shown superior to Satan's, for the magicians could perform only a few miracles and could go no further than God suffered, until they could not save themselves from the plagues.
The Egyptian magicians imitated the change in the water, and Pharaoh believed the plague was performed by magic and rejected it as proof of God's power. The warning and the miracle did not move Pharaoh to heed or humble himself; he hardened his heart and refused to obey and yield and let the people go, so the judgment visited him, and he became only more hostile toward the Israelites and increased his hatred of them.