Some geologists have characterized the area as alot of till on a bed of broken basalt - which acts like a layer of ball bearings. Which makes sense to me - you climb up the hillsides around that river and you can actually see the broken basalt. The rest is gravity, and the river wins in the end.

One regret I have is that I don't have a geologists eye for terrain - you walk a valley like the Nile with an experienced geologist and you could learn alot. My sympathies for the locals, they live in a hard spot now.