In order to really have this happen I imagine you'd want some sort of geologic PoD which exposed the Nubian Sandstone Aquifer - some sort of earthquake to bring the water to the surface could create a lake (or two, depending on location) - which in itself could have a great impact if the aquifer can manage to maintain itself.
Another factor to consider is how difficult it was to get past the cataracts in Egypt and Nubia, I don't see a drastic expansion of Egyptian civilisation or religion up the yellow river - not unless the added water flow makes the 1st-3rd cataracts easier to traverse. That increased water flow could also have drastic impacts on regions like faiyum/fayyum, and potentially (at a push) the quattara depression.
In addition, we could see trade caravans appear between the Yellow Nile, Lake Chad area, and even West Africa.
Almost without a doubt, we'd see a stronger sub-saharan africa, and potentially a smaller sahara desert.