Social Impact Assessment (SIA) functions as a strategic instrument for identifying and anticipating adverse social consequences throughout project life cycles. Utilizing a mixed-methods approach, this study investigates the social impacts of development initiatives on rural communities downstream of the Anahita Dam. Data were gathered through methodological triangulation, incorporating standardized questionnaires, structured interviews, and participatory field observations. The qualitative component engaged 35 key informants -including council members, village administrators, and local specialists- selected via snowball and criterion-based sampling techniques. In the quantitative phase, a stratified proportional sample of 295 household heads was drawn from a population of 1,435, using Bartlett et al.'s (2001) sampling table. Analytical methods included the Morris Model for quantitative data and qualitative content analysis with open and axial coding for qualitative data. The findings identified five principal impact categories: 1. demographic shifts, 2. uneven development, 3. diminished social capital, 4. heightened social instability, and 5. managerial-structural challenges. Application of the Morris Model demonstrated that Hajjiabad and Khorramabad villages achieved the highest and lowest rankings, respectively, across social development indicators. In conclusion, the findings demonstrate that the success of large-scale development projects such as the Anahita Dam necessitates a paradigm shift from a purely technical-economic perspective toward an integrated and accountable approach. To achieve sustainable and equitable development, it is imperative to formulate and implement evidence-based, targeted policies and planning frameworks that amplify positive impacts while minimizing adverse effects.