According to Exodus 7, the Nile remained changed for seven days after Moses and Aaron’s first plague.
In the biblical account, Aaron strikes the Nile with Moses’ staff in Pharaoh’s presence, and the river becomes blood. The water is said to become undrinkable, killing fish and producing a foul smell. The text then specifically states that seven days pass after the Nile is struck.
This event is traditionally counted as the first of the ten plagues of Egypt. It affects not only the main Nile but also canals, ponds, and stored water in wooden and stone containers. Egyptian magicians are described as imitating the transformation, although they do not remove its effects.
The seven-day detail is sometimes confused with the duration of later plagues or with the seven-day festival of Unleavened Bread. They are separate details in Exodus. The passage does not say that all ten plagues lasted seven days; it gives this duration only after the first plague.