Explanation and summary
The first plague of the 10 plagues that the God of the Israelites sent as judgment upon Egypt, through Aaron, was water turning to blood, as recorded in the book of Exodus. In this plague, the fish die, the river smells, and the people don't have drinkable, usable water. Pharaoh's magicians duplicate this plague. The Egyptian gods insulted by this plague include Khnum (guardian of the Nile), Hapi (spirit of the Nile), Orisis (god of the underworld), and perhaps others as well. The story is recorded in Exodus 7:14-25 (see text below).
The first of the plagues was brought about by the smiting of the water with the rod in the hand of Aaron, and it consisted in the defilement of the water so that it became as "blood." The waters were polluted and the fish died. Even the water in vessels which had been taken from the river became corrupt. The people were forced to get water only from wells in which the river water was filtered through the sand.
There are two Egyptian seasons when, at times, the water resembles blood. At the full Nile the water is sometimes of a reddish color, but at that season the water is quite potable and the fish do not die. But a similar phenomenon is witnessed sometimes at the time of the lowest Nile just before the rise begins. Then also the water sometimes becomes defiled and very red, so polluted that the fish die.
Biblical Text (KJV)
14 And the Lord said unto Moses, Pharaoh's heart is hardened, he refuseth to let the people go.
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.
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.
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.
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.
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.
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.
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.
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.
23 And Pharaoh turned and went into his house, neither did he set his heart to this also.
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. 25 And seven days were fulfilled, after that the Lord had smitten the river.
This latter time is evidently the time of the first plague. It would be some time in the month of May. The dreadful severity of the plague constituted the "wonder" in this first plague. The startling character of the plague is apparent when it is remembered that Egypt is the product of the Nile, the very soil being all brought down by it, and its irrigation being constantly dependent upon it.
Because of this it became one of the earliest and greatest of the gods (Breasted, Development of Religion and Thought in Egypt , 3-47; "Hymn to the Nile," Records of the Past , New Series, III, 46-54). The magicians imitated this plague with their enchantments. Their success may have been by means of sleight of hand or other devices of magic, as may be seen in the East today, with claim of supernatural aid, and as used in western lands for entertainment, as mere cleverness. Or it may be, as has been suggested, that they counted upon the continuance of the plague for at least a time, and so took advantage of the materials the "wonder" had provided.
International Standard Bible Encyclopedia, which is in the public domain (with minor edits).