Exodus 7:20

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.

Moses and Aaron did so, as the Lord commanded - i:e., they inflicted this first plague at the moment (Exodus 7:15) when the Nile god was receiving, or about to receive, the devoted homage of the Egyptian monarch. It was a blow to the system of that national worship, in which the Egyptians chiefly gloried.

Smote the waters. The act was symbolical: it was as if, by the energetic movement of the arm, Moses inflicted a wound upon the waters, and they were forthwith turned into blood (see the notes at Genesis 41:13). The colour of the water, which (previous to the rise of the river) is green, becomes at the commencement of that period red, from the immense quantity of slime which the impetuous tide rolls down from Sennaar, and yet, after it is filtered and the sediment is deposited, it is fit for use. But the red hue was supernaturally intensified, as appears from the universal destruction of the fish-an unprecedented occurrence, as well as by the loathsome state of the water.

There is no absolute neccessity for supposing that there was a chemical change of the water into a different fluid-in other words, that the water of the river was actually converted into blood; because the sanguineous hue was sufficient to symbolize the destruction of the enemies of Israel, and that was the design at once to remind them of the blood of the innocents shed in it, and to forewarn them of the retribution to be extracted (cf. 2 Kings 3:22; Joel 3:4). The miraculous character of this plague is manifested not only by the sudden alteration of the quality and colour of the river water, but by its occurrence consequent on the prediction, and the lifting of the rod of Moses.