It is known that the Nile was considered a deity by the Egyptians, as Rashi wrote (Exodus 7:17): "and the Egyptians worship the Nile," and this is sourced from the Midrash (Shemot Rabbah, Parsha 9, Siman 9): "Pharaoh and the Egyptians worship the Nile." The root of this abomination of deifying creations and worshipping them instead of accepting His Divine authority is rooted in ingratitude.

Specifically, we find this concept in the ingratitude of Egypt, as stated in the Midrash (Shemot Rabbah, Parsha 1, Siman 8) about the verse (Exodus 1:8) "who did not know Joseph" – "Today, he does not know Joseph, tomorrow he will say, 'I do not know the Lord' (Exodus 5:2)." Furthermore, in another Midrash (Sechel Tov, Buber, 1:8) it states, "who did not know Joseph" – meaning, he was ungrateful, as if he did not recognize Joseph, and forgot all his kindnesses, who brought all the money to Pharaoh's house (Genesis 41:14), and bought them and their land for Pharaoh (Genesis 47:20), and now he denied and betrayed him. Just as he denied Joseph, so did he deny his Creator, for Joseph had said, "What God is about to do, He has shown Pharaoh" (Genesis 41:28), and he said, "I do not know the Lord" (Exodus 5:2). Cursed are the wicked who are ungrateful, denying the one who does good for them, thus concludes the Midrash.

This is the language of the Yaarot Devash (Part 1, Drush 15) on this matter: "And this is the intention of the Midrash, 'Who did not know Joseph, today he does not know Joseph, tomorrow he will not know the Lord,' and this is as we wrote, because he was so ungrateful and made himself as if he did not know Joseph, who truly sustained all of Egypt, and without him, they would have already perished in the severe famine, and he gathered all the money coming to Egypt, and he was ungrateful to make himself as if he did not know. If so, this bad trait will cause him to ultimately deny the Lord and not recognize that all kindness comes from Him, being ungrateful and forgetting his Maker, and saying, 'The Nile is mine, and I have made it' (Ezekiel 29:3), and it comes out well that in the end he said, 'Who is the Lord that I should obey His voice?' Thus is the way of all those who walk in crooked paths, that they ultimately deny the Lord." Thus concludes the Yaarot Devash. We have already elaborated on this matter in Parshat Shemot.

Here we see how the root of the Egyptians' denial was in their ingratitude towards Joseph, and from their denial of Divine goodness, they came to complete denial of Him, and from there, the path to making the Nile a deity was short.

Accordingly, it is found that striking the Nile, whose purpose is to nullify its deity status, as Rashi brought: "and the Egyptians worship the Nile, therefore He struck their deity" – constitutes a correction for their ingratitude.