Exodus 7:15
Get thee unto Pharaoh in the morning; lo, he goeth out unto the water; and thou shalt stand by the river’s brink against he come; and the rod which was turned to a serpent shalt thou take in thine hand. Get thee unto Pharaoh in the morning; lo, he goeth out unto the water; and thou shalt stand by the river’s brink against he come; and the rod which was turned to a serpent shalt thou take in thine hand.
God commanded Moses to go to Pharaoh in the morning, to stand on the brink of the river to meet him as he came out to the water, and to take in his hand the rod that had been turned into a serpent. The same rod that had become a serpent was to be carried to the riverside encounter before the announcement of the first plague. Pharaoh's morning going out to the water was his regular, daily custom. It was punctual superstitious devotion, because the Nile was worshiped as a god and was an object of religious reverence.
The river was worshiped because it was Egypt's source of food and wealth. It supplied drinking water, watered the lands to make vegetation flourish, and liberally supplied fish and meat for their tables. Standing on its banks, Pharaoh offered praise and thanksgiving to the water, recounting the great good it did and declaring its great power. He declared that without it they could not exist, because their lands were watered by it and it supplied meat for their tables.
The meeting at the river was to deliver God's warning and renewed message. It gave Pharaoh advance notice of the judgment and another opportunity to let Israel go and serve God, with the plague on the waters to follow if he refused. There Moses and Aaron stretched the rod over the waters as the sign that God had sent them. They struck the water before Pharaoh and his officials and great men, beginning the first plague that turned the river to blood. All the fish died, the water became a putrid mass unusable for any purpose, and the plague lasted seven days.
The plague came upon the waters as one of Pharaoh's exalted gods. It showed that Egypt's revered river could neither protect nor save its worshipers and that Jehovah was supreme. The rod becoming a serpent and the river turned to blood left Pharaoh's hard heart unmoved and he still refused to yield. Because his magicians imitated the works he believed the miracles were performed by magic, and his hatred of the Israelites only increased.