Local and regional drivers of water quality in the Itaipu Reservoir (1985–2023)

Water quality in reservoirs is shaped by complex interactions between hydrological and landscape processes, yet the relative roles of mainstem and tributary dynamics remain unclear. Here, we leverage a four-decade dataset to examine the drivers of spatiotemporal water quality patterns in the Itaipu Reservoir, Brazil. We use linear mixed-effects models and statistical tests to relate water quality responses in the reservoir branches to sub-basin land-cover and hydrological conditions in the main river and tributaries. Our results show marked water quality heterogeneity influenced by sub-basin characteristics, while seasonal variability is primarily associated with tributary-scale hydrological conditions rather than with mainstem streamflow. Long-term trends of increasing nitrate and declining turbidity and chlorophyll-a are consistent with land-cover transitions, local interventions, and upstream cascade regulation. These findings highlight the importance of incorporating branch-scale dynamics into reservoir monitoring and management strategies.