Spatiotemporal changes in river network connectivity in the Nile River Basin due to hydropower dams

Dams are essential infrastructures, providing indispensable services, including hydropower generation, water for irrigation, and flood mitigation. However, their construction is primarily responsible for disrupting the natural river flows. The number of hydropower dams in the Nile River Basin has increased. However, little is known regarding the spatiotemporal variation in the river network fragmentation and the individual dam contributions. We used the Reach Connectivity Index (RCI) and network centrality measures to assess temporal and spatial changes in longitudinal connectivity. We evaluated the Equatorial Nile and Blue Nile, which are the major hotspots for present and future hydropower developments. Because we included proposed dams in the Equatorial Nile Basin (ENB), 101 dams from 1954 to 2035 and 19 in the Blue Nile Basin (BNB) from 1925 to 2035 were collated. The dams in the ENB have significantly increased over time, with a sharp increase recorded between 2010 and 2015. The mean RCI for the BNB, considering only impassable dams, ranged from 62.5% (SD = 12.5%) in 1925 to 21.35% (11.6%) in 2035. Similarly, in the ENB, the mean RCI for impassable dam scenarios ranged from 50.1% (SD = 2%) in 1954 to 18.1 (12.1%) in 2035. River reaches in the middle section of both basins were the most affected. The decline in the mean RCI was significantly higher in the ENB compared to BNB. Based on the individual dam contribution to the river network fragmentation, the proposed Karodobi dam in the BNB and Nabuyole dam in the ENB will significantly contribute to the fragmentation of the river networks. Therefore, the proposed dams, mainly Karodobi on BNB and Nabuyole Falls Dams, should not be constructed to avoid further reducing the longitudinal connectivity of each basin, respectively. Existing dams, including the Grand Ethiopian Renaissance Dam on the BNB and Kakono on the ENB, should have effective fish migratory corridors that allow the passage of fish either upstream or downstream.