| Summary: | Abstract Evaluating the spatial‐temporal dynamics in ecological risk and understanding its impact on water quality in reservoirs could optimize watershed land use and protect reservoir water quality. However, this impact remains elusive due to the lack of long‐term field data, the heterogeneity of land use, and scale effects. Therefore, Danjiangkou Reservoir area was selected as the study area, where rapid urban expansion and ecological conservation and restoration measures have significantly changed the ecological environment, altering the water quality. We investigated the spatial‐temporal changes of land use from 1990 to 2020 and evaluated how landscape ecological risk changed, as well as explored the impact of landscape ecological risk changes on water quality. The landscape ecological risk was calculated by landscape vulnerability and landscape disturbance (based on fragmentation, separation, and fractal dimension). The results indicated that the growth rate of water bodies surged (7.65 km2/a) and cropland experienced an apparent reduction (14.39%) over the past 30 years. These landscape changes decreased ecological risk, especially after the water transfer. The results revealed that the impacts of ecological risk on water quality were better explained at the riparian scale than at the reach and catchment scales. Specifically, the ecological risk was strongly relevant to dissolved oxygen, Turbidity, Cl− and NO3− ${{ ext{NO}}_{3}}^{-}$, and moderately correlated to Ca2+ and pH. While it was not correlated with total nitrogen, total phosphorus, and F−, possibly due to the irregular reservoir operations, occasional excessive use of fertilizers, and rock weathering. This study preliminarily discloses the impact of landscape ecological risk on water quality in a basin reservoir and provides a theoretical basis to take measures in advance for the sustainable development of a reservoir basin.
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