Integrated water quality indices as tools for evaluating small hydropower impacts

Robust water-quality assessment is critical to supporting evidence-based hydropower policy and compliance with river basin management objectives. This study evaluated the effectiveness and representativeness of selected water quality indices (WQIs, based on 25 water quality parameters), the Canadian Council of Ministers of the Environment WQI (CCME WQI), Oregon WQI (OWQI), Overall Index of Pollution (OIP), Dinius WQI (DWQI), and Universal WQI (UWQI), in assessing the impact of six run-of-river (RoR) hydropower plants (HPs) on the water quality of the Bóbr River in Poland. The CCME WQI values ranged from 37.86 to 57.48, corresponding predominantly to Class III water quality (69.74%). Overall, HP operations had only a minor influence on the variability of the analyzed indices, with downstream-to-upstream CCME WQI differences not exceeding 5%. Although no detectable changes in water quality were attributed to hydropower activity, substantial discrepancies were observed among the applied classification schemes (Polish – new and old – and Portuguese systems). Changes in WQI values were found to be more strongly influenced by reservoir-type HPs than by RoR HPs. For example, the observed variations for reservoir versus RoR HPs were: OIP (-18.81%/-1.59%), DWQI (-8.22%/0.84%), and UWQI (5.40%/0.12%). This study shows that RoR HPs have minimal impact on river water quality, with most changes falling within natural variability. However, discrepancies between WQIs and classification systems highlight the need for standardized assessment methods. These results advance understanding of hydropower–environment interactions and support the development of sustainable, climate-resilient hydropower management strategies through multi-index integration and cross-national comparisons of classification systems.