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, not merely be given a blood-red color through the presence of microscopic animals or particles of red clay, but actually be changed into blood, that the river throughout the length of Egypt would flow with the liquid which commonly pulses through the arteries and veins of men and beasts.