Approaches to fill the Grand Ethiopian Renaissance Dam (GERD) are simulated with various degrees of coordination between the co-riparian countries and evaluated using a wide range of historical hydrological conditions. The modelled effects on water availability and energy generation demonstrate that water diversions in Sudan can be largely assured through adaptations to Sudanese reservoir operations. The risks to Egyptian users and energy generation can be minimised through combinations of sufficient agreed annual releases from the GERD, a drought management policy for the High Aswan Dam, and a basin-wide cooperative agreement that protects the elevation of Lake Nasser.