Monitoring and analyzing reservoir water quality are crucial for ecological and environmental strategies. This study examines the differences between the Water Quality Indices (WQI) of Indonesia (WQI-INA), Malaysia (DOE-Malaysia), Oregon (WQI-Oregon), and Italy (PI-Prati), considering the tropical climate of Indonesia and Malaysia, and contrasting with the temperate climate of Oregon and the Mediterranean climate of Italy. Using data from the Sutami Reservoir in Indonesia from 2015 to 2021, the study assesses water quality parameters such as BOD, COD, DO, NH3, NO3, total suspended solids, total phosphorus (TP), and pH at various depths (0.3, 5, and 10 meters) across three sites (upstream, midstream, downstream). The results show varied classifications: WQI-INA rated the reservoir as "pretty good," WQI-Oregon as "poor," and both DOE-Malaysia and PI-Prati as "slightly polluted." Spatial analysis of vertical distribution revealed "slightly polluted" (WQI-INA), "moderately polluted" (WQI-Oregon and PI-Prati), and "heavily polluted" (DOE-Malaysia). These discrepancies arise from differences in local environmental standards, regulatory requirements, and specific water quality concerns included in each index. The Sutami Reservoir water quality status was declared using WQI-INA, however, other methods were employed to simulate the worst-case scenarios and inform preventive actions. The Sutami Reservoir hyper-eutrophic conditions indicate a pollution load capacity of 65.22 tons per year. Critical parameters such as pH and TP highlight significant water quality and pollution issues. Recommendations for water quality treatment under heavy pollution conditions include applying 12.09 - 120.87 tons of lime, using 78.75 - 393.75 kg of chemical buffers, oxygen diffusion, controlling riparian vegetation over 2.37 - 23.7 hectares, dredging 37,100 m3 of sediment per year, and reducing nutrient sources through community involvement.