Construction of Black Hole Dams (reservoir volume in billion m3) traps flood, sediment and its impacts on the downstream countries is the most controversial issue for Transboundary Rivers. This study aims to understand the water availability and its spatial and temporal distribution in the four sub basins of Eastern Nile Basin (ENB). The study provides assessment of an overview of Ethiopia's plans to build a mega dam on the Blue Nile so-called Grand Ethiopian Renaissance Dam (GERD). As the Ethiopian Highlands are the main source of sediment, the river systems draining this region have witnessed severe sediment management problems. In this regard, an operational water balance model Soil and Water Assessment Tool (SWAT) has been set up for this basin. From simulation results, it has been found that the annual average flow at Aswan High Dam (AHD) is reduced to maximum 48% from the base condition. The response of AHD to reservoir sedimentations and the reduced sediment yield is questionable. To address and cope with this reduced flow, it is essential to design adaption measures accordingly. Coordinated sediment management and dam operation are highly recommended for cascade dams along the Blue Nile. © 2014 Taylor & Francis Group, London.