The grand Renaissance Dam in Ethiopia will stop silt reaching the white Nile- during the high flood this used to colour the white nile red with the sediment. It was believed that a heavy flood was the source of the Biblical account of Moses' curse - in which God turned the Nile 'red with blood'. Well the Aswan dam stopped the silt getting to Cairo and this Ethiopian dam will stop it getting outside Ethiopia. Which means the grand Renaissance dam will silt up pretty fast and the Aswan and other dams will not. Evaporation from the new dam will cost Egypt 2% less water- it can stand this easily. The real damage will be to ordinary Nile dwelling Ethiopians. They wil not benefit from the electricity- 80% are off the grid and no grid is being proposed- the electricity is for export to cities in Kenya and Sudan.
For more general background this wiki piece is good; http://en.wikipedia.org/wiki/Grand_Ethiopian_Renaissance_Dam