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.

Jamieson-Fausset-Brown Bible Commentary

17-21. Aaron lifted up the rod and smote the waters, &c.-Whether the water was changed into real blood, or only the appearance of it (and Omnipotence could effect the one as easily as the other), this was a severe calamity. How great must have been the disappointment and disgust throughout the land when the river became of a blood red color, of which they had a national abhorrence; their favorite beverage became a nauseous draught, and the fish, which formed so large an article of food, were destroyed. [See on [17]Nu 11:5.] The immense scale on which the plague was inflicted is seen by its extending to "the streams," or branches of the Nile-to the "rivers," the canals, the "ponds" and "pools," that which is left after an overflow, the reservoirs, and the many domestic vessels in which the Nile water was kept to filter. And accordingly the sufferings of the people from thirst must have been severe. Nothing could more humble the pride of Egypt than this dishonor brought on their national god.

Matthew Henry's Concise Commentary

7:14-25 Here is the first of the ten plagues, the turning of the water into blood. It was a dreadful plague. The sight of such vast rolling streams of blood could not but strike horror. Nothing is more common than water: so wisely has Providence ordered it, and so kindly, that what is so needful and serviceable to the comfort of human life, should be cheap and almost every where to be had; but now the Egyptians must either drink blood, or die for thirst. Egypt was a pleasant land, but the dead fish and blood now rendered it very unpleasant. It was a righteous plague, and justly sent upon the Egyptians; for Nile, the river of Egypt, was their idol. That creature which we idolize, God justly takes from us, or makes bitter to us. They had stained the river with the blood of the Hebrews' children, and now God made that river all blood. Never any thirsted after blood, but sooner or later they had enough of it. It was a significant plague; Egypt had great dependence upon their river, Zec 14:18; so that in smiting the river, they were warned of the destruction of all the produce of their country. The love of Christ to his disciples changes all their common mercies into spiritual blessings; the anger of God towards his enemies, renders their most valued advantages a curse and a misery to them. Aaron is to summon the plague by smiting the river with his rod. It was done in the sight of Pharaoh and his attendants, for God's true miracles were not performed as Satan's lying wonders; truth seeks no corners. See the almighty power of God. Every creature is that to us which he makes it to be water or blood. See what changes we may meet with in the things of this world; what is always vain, may soon become vexatious. See what mischievous work sin makes. If the things that have been our comforts prove our crosses, we must thank ourselves. It is sin that turns our waters into blood. The plague continued seven days; and in all that time Pharaoh's proud heart would not let him desire Moses to pray for the removal of it. Thus the hypocrites in heart heap up wrath. No wonder that God's anger is not turned away, but that his hand is stretched out still.