exodus 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. - Free Bible Online

      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 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.

JFB.