As Pharaoh would not lay the warning to heart, the plague announced by Moses was let loose upon him and his people--the waters were turned into blood. It is a well-known proverb, "Beat the idols, and the priests are in terror." God smote the river Nile, which the Egyptians worshipped as their god, in order to terrify Pharaoh and his people and force them to do the Divine will.
To produce the plague, Aaron took his rod, and stretched out his hand over the waters of Egypt. Moses had no part in performing the miracle, for God had said to him, "The water that watched over thy safety when thou wast exposed in the Nile, shall not suffer harm through thee."