Exodus 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. The Nile was the one source of fertility and life in Egypt, and therefore the Lord indicated by this miracle that it was an easy matter for Him to change all the blessings of the country into curses. The very Nile to which the Egyptians gave divine honor was subject to the command of the God of the Hebrews, and this fact was to be impressed upon them forcibly.