What did the Aswan High Dam stop happening?

The annual flood. For at least five thousand years the Nile rose every summer, spread across the valley, and left a layer of fertile silt behind - which is the entire reason a civilisation existed in the middle of a desert. The Egyptian calendar, tax system and land surveying were all built around it, because the flood erased field boundaries and they had to be measured out again each year.

The dam, completed in 1970, ended that in exchange for things Egypt badly wanted: reliable irrigation for three harvests a year instead of one, protection from both drought and catastrophic flood, and about half the country's electricity at the time. Those were real gains and it is not a straightforward mistake.

The costs came with them. The silt now settles in Lake Nasser instead of on the fields, so Egyptian agriculture depends on manufactured fertiliser; the delta, no longer being resupplied, is eroding into the Mediterranean and salt is moving into the groundwater; the sardine fishery off the coast collapsed when the nutrient pulse stopped. Around 100,000 Nubian people were displaced and their homeland drowned, and the temples at Abu Simbel had to be cut into blocks and rebuilt sixty metres higher up.