"So the LORD said to Moses: 'Pharaoh's heart is hard; he refuses to let the people go. Go to Pharaoh in the morning when he goes out to the water, and you shall stand by the river's bank to meet him; and the rod which was turned to a serpent you shall take in your hand. And you shall say to him, "The LORD God of the Hebrews has sent me to you, saying, 'Let My people go, that they may serve Me in the wilderness'; but indeed, until now you would not hear! Thus says the LORD: 'By this you shall know that I am the LORD. Behold, I will strike the waters which are in the river with the rod that is in my hand, and they shall be turned to blood. And the fish that are in the river shall die, the river shall stink, and the Egyptians will loathe to drink the water of the river'"'" (Exo. 7:14-18).
Because Pharaoh's heart was hard and would become increasingly so, and since he was not persuaded to action by the first sign (i.e., Moses' staff turning into a snake), God would bring miraculous plagues against Egypt in close succession and destroy the land. These plagues were selected by God to prove that Jehovah was supreme in their land, having unrestricted freedom and power. These plagues would suddenly burst on the land, being announced and "produced" by Moses and Aaron.
The first plague against Egypt and her gods would strike the Nile River. The Nile was the heartbeat of Egypt; commerce and crops depended upon it. This plague would be an attack against Khnum, the guardian god of the Nile, represented as a human being with a ram's head, and also against the god Hapi who was the spirit or dynamic essence of the river. The water would become undrinkable and the living creatures therein would perish. The stench would be overwhelming.
"Then the LORD spoke to Moses, 'Say to Aaron, "Take your rod and stretch out your hand over the waters of Egypt, over their streams, over their rivers, over their ponds, and over all their pools of water, that they may become blood. And there shall be blood throughout all the land of Egypt, both in buckets of wood and pitchers of stone."' And Moses and Aaron did so, just as the LORD commanded. So he lifted up the rod and struck 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. The fish that were in the river died, the river stank, and the Egyptians could not drink the water of the river. So there was blood throughout all the land of Egypt. Then the magicians of Egypt did so with their enchantments; and Pharaoh's heart grew hard, and he did not heed them, as the LORD had said. And Pharaoh turned and went into his house. Neither was his heart moved by this. So all the Egyptians dug all around the river for water to drink, because they could not drink the water of the river. And seven days passed after the LORD had struck the river" (7:19-25).
Where were Khnum and Hapi when this attack was brought against the Nile? Were they not as powerful as Jehovah? These questions would weigh on the Egyptians mind's as they struggled to find suitable drinking water and endured the stench for at least a week. Pharaoh's mind is unchanged, however, especially since his magicians were able to imitate the miracle (though on a much smaller scale, obviously). One might ask: Where did the magicians get water to turn into "blood"? Presumably there was a limited supply of water available in buckets or pitchers that held water drawn prior to the plague. If there was some other pure source for them to acquire water, there would not have been a need to dig wells.