What does Exodus 7:19 mean?
From the very beginning of his call (Exodus 3:4–6), Moses expressed self-doubt (Exodus 4:10, 13). God appointed Moses' brother, Aaron, to be the official public speaker of the mission (Exodus 4:14–16). Here, Aaron acts according to the Lord's directions as passed down through Moses. This, in turn, is a response to Pharaoh's stubborn refusal (Exodus 7:14) to release the Israelites from slavery (Exodus 1:11–14; 5:1–4). God will demonstrate His authority by turning all water into blood (Exodus 7:17).This verse notes that this first plague (Exodus 3:19–20) will affect more than just the waters of the Nile River. Reservoirs, streams, irrigation canals, and ponds will be corrupted. Even water in manmade vessels such as jars and bowls will be transformed. That wide influence supports the Lord's message in several ways. It will leave the Egyptians no choice but to scrabble for water (Exodus 7:24–25). It removes any room to excuse pollution of a single source as nothing more than human sabotage. The same detail also complicates the theory that God used a natural phenomenon to change the color and odor of the river (Exodus 7:21).
As with turning staves into snakes (Exodus 7:8–11), Egyptian court magicians will convince their king that they can do the same thing (Exodus 7:22). Whether this is by actual supernatural power, or stage magic, the result will be the same: Pharaoh will remain obstinate (Exodus 7:23).