Abstract
<title>Abstract</title> <p>Hydrological flow alteration is widely recognized as a critical approach for assessing river flow regimes and associated ecological health. Although this approach remains relatively new in Ethiopia, it holds significant potential for guiding future river management and conservation efforts. This study aimed to evaluate the ecological health of the Gilgel Abay River using hydrological flow alteration metrics. Flow alterations were analyzed using the Indicators of Hydrologic Alteration (IHA) model based on daily discharge data collected over a 42-year period (1979–2020). The results revealed a declining trend in both high-flow and low-flow durations, particularly for the 1-day and 90-day flow indices. Analysis of environmental flow components showed that only four major flood events occurred during the study period (1980, 1986, 1988, and 1994), with no large floods recorded after 1995. In contrast, the frequency of extremely low-flow events increased after 2002, while small flood events have been absent since 2008. The flow duration curve indicated that stream flow equaled or exceeded 15.81 m³/s for 95% of the time, reflecting persistent low-flow conditions in the study area. Overall, the findings demonstrate substantial ecological degradation resulting from flow regime alteration, highlighting the urgent need for sustainable flow management strategies in the basin.</p>