One reads about the “cataracts of the Nile” in history books (always with ordinals, like “the first cataract,” “the second cataract” and so forth), but until I saw the first cataract just south of Aswan it was hard for me to picture them–I thought of something like a series of little Niagaras, walls of water blocking passage north. Now, the first cataract currently lies just on the other side of dams (the old Aswan Dam and the High Dam), and so the sort of rapids or rush of water is no longer there, but seeing the rocks in the water at least helped me visualized what these cataracts are–places that, due to topography, are unnavigable.
One of the wonders, to me, of Egyptian civilization is now much it stayed put along the Nile. There were of course times when Egypt controlled south toward Nubia, west toward Libya and east toward the Levant, but despite its incredible wealth and advancement it was never an expansive empire, instead being controlled by the Persians, the Greeks (or Macedonians), the Romans, the Arabs and the Ottomans. One basic reason for this, I suppose, is that Egyptian civilization was centered on the Nile, and perhaps they saw no reason to stray far from what they saw as the source of all life. In periods of ascendancy Egypt did control regions further south, into Nubia, but the cataracts–areas where the Nile was not navigable–presented a barrier. Were it not for the cataracts, perhaps Egyptian civilization would have traveled all the way to Lakes Tana and Victoria, deep into sub-Saharan Africa.