The Nile flows through Cairo before reaching the Mediterranean Sea. It is formed by two major tributaries, the White Nile and the Blue Nile, which meet at Khartoum in Sudan. From there, the river runs north through Egypt and spreads into a broad delta.
Cairo stands on the Nile's eastern bank and islands near the southern part of the delta. The river has supported settlements, farming, transport, and states in northeastern Africa for thousands of years. Ancient Egyptian society depended heavily on its seasonal flooding, which deposited fertile silt across the floodplain.
The Aswan High Dam, completed in 1970, greatly reduced the natural floods and created Lake Nasser. It improved control of irrigation and electricity generation, but it also reduced the amount of fresh silt reaching downstream farmland.
The Nile is commonly called the world's longest river, although some measurements and definitions produce debate with the Amazon. Regardless of that comparison, its role in Egyptian history and Cairo's geography is undisputed.