Why the first big cities grew by rivers, and where the Nile’s summer flood comes from.
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Some of the world’s first big cities and kingdoms grew up beside four rivers: the Tigris and Euphrates, the Nile, the Indus and the Yellow River. The oldest were in Mesopotamia (now Iraq), where people were writing more than 5,000 years ago. Why build next to a river?
The Nile is the longest river in the world. In Egypt it flows through the desert, in the dry stripe near 30° north from the Deserts lesson. But look at the green strip of farms along it: 95 out of every 100 Egyptians live within a few kilometres of the river. (The rivers, farms and lake in this lesson are drawn simpler and wider than they really are.)
Every summer, the Nile rose and spilled over its banks, flooding the valley. When the water went down, it left a layer of dark, rich mud called silt. It was like free fertiliser, and farmers grew wheat and barley in it.
Egypt is desert, so where does all that water come from? From far to the south. The White Nile starts near Lake Victoria, and the Blue Nile starts at Lake Tana, in the mountains of Ethiopia. They meet at Khartoum, and the Blue Nile brings most of the water.
In 1970, Egypt finished the Aswan High Dam, a giant wall of earth and rock across the Nile. Tap the button to build it. What happens to the flood?
Egypt is desert, but every summer before 1970 the Nile flooded.
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The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).