<p>Understanding the spatiotemporal relationships between land use change and river NH₃-N and TP concentrations is essential for sustainable watershed management in ecologically fragile areas. This study focused on the Min County section of the Taohe River Basin, a typical ecologically fragile area in China. By coupling the PLUS and InfoWorks ICM models, this study investigated the spatiotemporal dynamics of land use and water quality and their interactions. Three scenarios, namely Natural Development, Grain for Green Program (GFGP), and Urban Expansion, were established to simulate future trends. Results showed that cultivated land remained generally stable during the study period. Natural factors, particularly precipitation and topography, primarily drove the expansion of grassland and water bodies, whereas socio-economic factors, especially policy implementation and urban expansion, dominated the growth of construction land and forest land, resulting in substantial forest land expansion, particularly during 2000–2010 owing to large-scale forestry and grassland restoration projects. Riverine NH₃-N and TP concentrations exhibited pronounced seasonal and spatial heterogeneity. Among different land-use transitions, the conversion of cultivated land and grassland to forest land contributed most significantly to water quality improvement. Scenario simulation results indicated that the GFGP scenario could reduce riverine NH₃-N and TP concentrations to the greatest extent, but would lead to runoff reduction and cultivated land loss. Therefore, a moderate GFGP strategy may better balance ecological restoration, agricultural production, and watershed sustainability. These findings provide scientific support for land use optimization and water environment management in ecologically fragile areas.</p>