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Abstract The hydrological effects of climate change are documented in many regions; however, climate-driven impacts to the source and transport of river nutrients remain poorly understood. Understanding the factors controlling nutrient dynamics across river systems is critical to preserve ecosystem function yet challenging given the complexity of landscape and climate interactions. Here, we harness a large regional dataset of nitrate (NO₃^–) yield, concentration, and isotopic composition (δ¹⁵N and δ¹⁸O) to evaluate the strength of hydroclimate and landscape variables in controlling the seasonal source and transport of NO₃^–. We show that hydroclimate strongly influenced the seasonality of river NO₃^–, producing distinct, source-dependent NO₃^–regimes across rivers from two mountain ranges. Riverine responses to hydroclimate were also constrained by watershed-scale topographic features, demonstrating that while regional climate strongly influences the timing of river NO₃^–transport, watershed topography plays a distinct role in mediating the sensitivity of river NO₃^–dynamics to future change.