Nutrients regulate algal growth, drive microbial processes, and determine food web structure. Despite major alterations to nutrient dynamics, temporal and spatial patterns of nutrient retention are poorly understood in large rivers in part due to limited sample records. On the dammed, nutrient-polluted Klamath River (CA), the Karuk and Yurok Tribes have collected over two decades of biweekly nutrient (N and P) samples, allowing us to model daily nutrient concentration and loads from 2005-2023 and then calculate monthly, reach-scale (25-65 km) nutrient retention and release. P was high compared to N in all reaches of the Klamath River with N:P << 16:1, and nutrient concentrations generally decreased while loads increased in a downriver direction due to tributary accretions. Seasonal variation was high, spanning two orders of magnitude, suggesting that baseflow metrics will severely underestimate annual retention. Nutrients also varied longitudinally, with dams and major tributary confluences shifting the relationship between discharge and nutrient retention. In stable upriver reaches, higher discharge suppressed retention, while in larger downriver reaches, it enhanced retention via delivery of the limiting nutrient N. Other differences aligned with diverse hydrology, geology, and autotrophic assemblages. Nutrient dynamics in the Klamath River help us understand more broadly how dams and nutrient pollution lead to nuances in eutrophication patterns across time and space, and how large-scale dam removal may impact ecosystems from the bottom up. This thorough investigation into nutrient dynamics in a large, 7th order river was only possible due to long-term, place-based Tribal datasets, which offer insight into how nutrient pollution and dams impact the aquatic ecology of large rivers more broadly.