"Behold, out of the River there emerged seven cows, of pleasing appearance and healthy flesh, and they were grazing in the marshland." (Bereishit 41:2)
The Zohar discusses Pharaoh's dream, in which he saw seven fat cows emerging from the river, which were swallowed by seven emaciated cows. He then saw seven sheaves of full healthy stalks of wheat which were swallowed up by seven sheaves of withered and emaciated wheat.
The only image in Pharaoh's dream that is not interpreted by Yosef is the river. The river is taken to mean the Nile, which was the source of sustenance for all of Egypt.
What is the meaning of the river in Pharaoh's dream?
It is the river from which all of the levels below are blessed, because this river is drawn down and goes forth to irrigate and nourish them all - Yosef is the river for whom all of Egypt was blessed.
(In Kabbalah, the river that sustains all of the lower levels is the attribute of Yosef - the sefira of yesod of the world of Atzilut, which draws down the waters of the world of Atzilut into Beriya.)
The Jewish people came down to Egypt to elevate the holy sparks that fell into the physical world when the world of Tohu shattered. This is what was being shown to Yosef in Pharaoh's dreams.