The river turned to blood in the first plague of Egypt was the Nile.
In the Book of Exodus, Moses is commanded to strike the Nile with his staff. The water becomes blood, killing its fish and making it undrinkable. The plague extends beyond the main river: water in Egyptian vessels is also affected, while the Egyptians dig beside the river to find drinkable water.
This is the first of the ten plagues, which the biblical narrative presents as signs compelling Pharaoh to release the Israelites from slavery. The plagues are retold during the Passover Seder, connecting the river episode with the Exodus story.
A common mix-up is confusing the Nile with other major rivers of the ancient Near East, such as the Tigris or Euphrates. The Nile is unmistakable in Exodus because Egypt’s agriculture, cities and religion were closely tied to it. Some modern discussions compare the story with the Ipuwer Papyrus, but that text is not accepted as a straightforward historical record of the biblical plague.