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Water flow

The flow of the Blue Nile reaches maximum volume in the rainy season from June to September, when it supplies 80 – 86% of the water of the Nile proper. The river was a major source of the flooding of the Nile in Egypt that contributed to the fertility of the Nile Valley and the consequent rise of Ancient Egypt and Egyptian mythology. With the completion of the Aswan Dam in 1970, these floods stopped occurring in lower Egypt. During the summer monsoon season, the Blue Nile floods erode a vast amount of fertile soil from the Ethiopian Highlands and carry it downstream as silt, turning the water dark brown or almost black.

The Blue Nile is vital to the livelihood of Egypt: as the most significant tributary of the Nile, it contributes over 85% of the Nile's streamflow. Though shorter than the White Nile, 59% of the water that reaches Egypt originates from the Ethiopian highlands via the Blue Nile. The river is also an important resource for Sudan, where the Roseires Dam and Sennar Dam contribute to the 80% of the country's electricity generation from hydropower. These dams also help irrigate the Gezira Scheme, which is most famous for its high-quality cotton, as well as wheat and animal feed crop production in the area.

In November 2012, Ethiopia began construction of the Grand Ethiopian Renaissance Dam, a 6000-megawatt hydroelectric dam on the river. The dam is expected to be a boost for the Ethiopian economy. Sudan and Egypt, however, voiced their concerns over a potential reduction in water available. Electricity generation began in February 2022.