At the height of the Arab Spring in March 2011 when Egypt was consumed by domestic unrest, Ethiopia announced it would soon begin construction on a hydroelectric dam along the Blue Nile River, one of two main tributaries for the Nile. The Grand Ethiopian Renaissance Dam, which will be one of the 10 largest dams in the world at its estimated completion in 2017, will likely interrupt or limit the flow of water to Egypt and Sudan for several years and could permanently alter the amount of water those countries are able to draw from the river. For decades, Cairo has held sole authority over Nile water allocation under a colonial-era agreement signed in 1959 by Sudan, Egypt and the United Kingdom.