Exodus 7:19
And the LORD spake unto Moses, Say unto Aaron, Take thy rod, and stretch out thine hand upon the waters of Egypt, upon their streams, upon their rivers, and upon their ponds, and upon all their pools of water, that they may become blood; and that there may be blood throughout all the land of Egypt, both in vessels of wood, and in vessels of stone.
God required that Moses and Aaron faithfully tell Pharaoh beforehand the nature and effect of the stroke upon the waters, so that he would have the privilege of averting it by letting Israel go to serve and sacrifice to God in the wilderness, with continued refusal bringing judgment upon him. At His command Aaron stretched out the rod and struck the waters openly in the sight of Pharaoh and his officials, servants, and great men. The stroke was the first plague, and it fell on the waters of Egypt throughout all the land, upon the Nile, upon streams, rivers, ponds, pools, household cisterns and water kept in houses, both in vessels of wood and in vessels of stone.
The Egyptians worshiped the Nile and its waters as gods and gave to them the devotion belonging to God alone, because they depended on them for drinking water, irrigation, food and wealth, trusting them to quench thirst, water lands, make vegetation flourish, and supply fish for food. Pharaoh himself went to the river every morning and stood on its banks offering praise and thanksgiving to the water, declaring the good it did and its great power and that without it they could not exist. Turning the Nile and Egypt's other waters and their fish, which were revered as sacred objects of worship, to blood struck one of Pharaoh's exalted gods, exposed the idols' inability to protect themselves or help their worshipers, and showed Jehovah's supremacy over them.
The water they praised for its power was turned to blood before them, became a putrid, foul and offensive mass, the fish died, and the people could not use the water for any purpose. The plague upon the waters continued for seven days. The people had to dig wells to obtain water. The Egyptian magicians imitated the change with water drawn from wells, which led Pharaoh to believe the miracles were performed by magic.
Pharaoh would not listen but despised their words, remained unmoved, hardened his heart, did not humble himself, and refused to yield and let the people go, yet he had no power to harm Moses and Aaron. The judgment did not soften Pharaoh's hard heart but only increased his hatred of the Israelites. The turning of the water to blood was a judgment from God and a miraculous interposition in behalf of Israel, one of the calamities visited on the Egyptians for persecuting His chosen people, displaying His protecting power and His hand staying the evils surrounding Israel. Israel's forgetting this miracle when walled in at the Red Sea showed impatience under test, lack of trust in God, reproach against Moses, and failure to let past deliverance sustain faith in present peril.