Where do gorges come from?

I'm not sure if this is the right NG to ask this question, and this may
seem a remarkably ignorant question, but that never stopped me before.

On television, they've just shown the Blue Nile forming a waterfall in
a massive gorge. The land looks, given the aerial shot, amazingly flat
until there's a sudden "fresh looking" (sharp edges) massive gorge.
What forces could cut such a massive hole in the earth? I had a look,
and Lake Tana where the Blue Nile rises, seems well away from the great
rift valley.

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