The First plague: Waters in the river turned into blood. Exo 7:15
Get thee unto Pharaoh in the morning; lo, he goeth out unto the water; and thou shalt stand by the river's brink against he come; and the rod which was turned to a serpent shalt thou take in thine hand.
Exo 7:16 And thou shalt say unto him, The LORD God of the Hebrews hath sent me unto thee, saying, Let my people go, that they may serve me in the wilderness: and, behold, hitherto thou wouldest not hear.
Exo 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.
Exo 7:18 And the fish that is in the river shall die, and the river shall stink; and the Egyptians shall lothe to drink of the water of the river.
Exo 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.
Exo 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.
Exo 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.
Exo 7:22 And the magicians of Egypt did so with their enchantments: and Pharaoh's heart was hardened, neither did he hearken unto them; as the LORD had said.
Exo 7:23 And Pharaoh turned and went into his house, neither did he set his heart to this also.
Exo 7:24 And all the Egyptians digged round about the river for water to drink; for they could not drink of the water of the river.
Exo 7:25 And seven days were fulfilled, after that the LORD had smitten the river.
Every morning Pharaoh would go down to the Nile to bless its life- giving waters, purify himself in them and give thanks to hapi, the god that watches over the waters and fish. The fact that the court magicians duplicated the miracle that Moses performed only gave Pharaoh a false sense of security against the power of G-d. The Egyptian magicians mocked G-D and Moses. For this, the punishment was expanded to all the waters. Hapi was one of the most important gods they had, along with rem (the Egyptian fish god, or the weeping god), and that is why this plague was the first thing G-D had chosen. This god was known as the personification of Ra's tears. Egypt had putrefying bodies of fish and people throughout the land for a week. Without water, no one had the strength to remove the dead to tombs, or to mummify or clean anything. G-D did have mercy on the Egyptians by changing the water back. This would also show that their god over the Nile river was dead. and the river of life was changed to a river of death much like it was when Pharaoh ordered the young male children of Israel to be tossed in the Nile.
Exo 1:22 And Pharaoh charged all his people, saying, Every son that is born ye shall cast into the river, and every daughter ye shal save alive.
Picture from here