Despite increasing concern about rapid land-use change and eutrophication of temperate mountain ecosystems, uncertainty persists about how stream ecosystems will retain, process, and export elevated concentrations of reactive nitrogen (N). Mountain watersheds that exhibit spatial heterogeneity in land use and large temporal variation in biotic demand for N can provide the opportunity to address these uncertainties, particularly where long-term data exist. We quantified spatial and temporal variation in dissolved N in tributary outlets that contribute to the West Fork of the Gallatin River, which drains a rapidly developing ski resort community in Big Sky, MT, USA. We collected bi-weekly stream grab samples and deployed continuous nitrate sensors at 3 sites from October 2024 to 2025, varying in development intensity. We then paired these data with comparable data from 2005 to estimate changes in concentration and N loading over a period during which housing development nearly doubled (~3300 to 5300 structures). Preliminary results suggest that rapid development has continued to elevate N concentrations and export, particularly in highly developed streams, suggesting early signs of N saturation driven by increased wastewater inputs. Streams draining high-development landscapes exhibit four-fold higher average N concentrations and a much higher magnitude of diel fluctuations relative to nearby pristine stream sites. Although enriched tributaries had higher algal biomass than pristine ones, elevated concentrations throughout the year suggest reduced ability of these ecosystems to retain excess N. Without further efforts to curtail N inputs, our data suggest that continued enrichment of downstream ecosystems is inevitable. By comparing water quality data across 20 years of development and leveraging continuous diel water quality signals from sites with varying levels of development intensity, our research helps to deepen our understanding of how temperate mountain streams respond to long-term land-use change.