Because Moses complains (Ex. 4:8) that he is not a speaker, Aaron is appointed as his spokesman (Ex. 7:1-2: according to the text Aaron is to be Moses’ navi, “prophet”, i.e. interpreter). Moses is to give Aaron the message; Aaron is to turn it into the appropriate rhetoric.
When the Ten Plagues take place, however, Aaron suddenly becomes more than a mere mouthpiece and it is he, not Moses, who brings about at least some of the plagues.
Who smites the waters so that they turn to blood? Who stretches his hands over the waters and produces the frogs? Who smites the dust so that it produces lice? Not Moses but Aaron.
What stops Moses from bringing about these plagues?
According to the rabbinic tradition, it is ethics and decency. As a baby Moses was protected by the River Nile and it would not be right for him to harm the river that was good to him (Rashi on Ex. 7:19).
Yes, the Egyptians were wicked. Yes, punishment had to come upon the land. But means matter, not just ends. One must not repay good with evil.
If the Nile needs to be affected and afflicted, let it be by means of someone else who does not owe the river a debt of gratitude.