Aleksei, thanks for noting the error on the quiz question. I’ll ask the web people to fix that.
You might be surprised how much water could be diverted from even fairly large rivers by digging channels for irrigation and the like. The Persians famously changed the course of the entire Euphrates during one of their military campaigns. During years of drought, river levels could get fairly low, and near the mouth of the river this could lead to increased salinity in the water to the point where it might become problematic for drinking or irrigation.
The plausibility of the “challenge/response” theory for the rise of civilization in Mesopotamia is based on the major coordination needed to construct levees along the rivers, which flooded violently at various times. It doesn’t matter how good the farmland is if it’s under water. No, I don’t emotionally like the theory. The “prosperity” theory appears to fit Egypt’s circumstances better on the face of things.