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Excessive phosphorus (TP) and nitrogen (TN) inputs from the Red-Assiniboine River Basin (RARB) have been linked to eutrophication of Lake Winnipeg; therefore, it is important for the management of water resources to understand where and from what sources these nutrients originate. The RARB straddles the Canada-United States border and includes portions of two provinces and three states. This study represents the first binationally focused application of SPAtially Referenced Regressions on Watershed attributes (SPARROW) models to estimate loads and sources of TP and TN by jurisdiction and basin at multiple spatial scales. Major hurdles overcome to develop these models included: (1) harmonization of geospatial data sets, particularly construction of a contiguous stream network; and (2) use of novel calibration steps to accommodate limitations in spatial variability across the model extent and in the number of calibration sites. Using nutrient inputs for a 2002 base year, a RARB TP SPARROW model was calibrated that included inputs from agriculture, forests and wetlands, wastewater treatment plants (WWTPs) and stream channels, and a TN model was calibrated that included inputs from agriculture, WWTPs and atmospheric deposition. At the RARB outlet, downstream from Winnipeg, Manitoba, the majority of the delivered TP and TN came from the Red River Basin (90%), followed by the Upper Assiniboine River and Souris River basins. Agriculture was the single most important TP and TN source for each major basin, province and state. In general, stream channels (historically deposited nutrients and from bank erosion) were the second most important source of TP. Performance metrics for the RARB SPARROW model are similarly robust compared to other, larger US SPARROW models making it a potentially useful tool to address questions of where nutrients originate and their relative contributions to loads delivered to Lake Winnipeg.Les apports excessifs en phosphore (PT) et en azote (AT) en provenance des bassins de la rivière Rouge et de la rivière Assiniboine (BRRA) ont été liés à l'eutrophisation du lac Winnipeg; il est par conséquent important pour la gestion de comprendre où et de quelles sources proviennent les apports. Les BRRA chevauchent la frontière canado-américaine et englobent partiellement deux provinces et trois États. La présente étude représente la première application binationale des modèles SPARROW (SPAtially Referenced Regressions On Watershed attributes, ou régressions par coordonnées spatiales appliquées aux bassins versants) pour estimer les charges et les sources de PT et d'AT de chaque territoire et bassin versant à de multiples échelles spatiales. Des obstacles majeurs ont dû être contournés pour mettre au point ces modèles, dont: (1) l'harmonisation d'ensembles de données géospatiales, en particulier la construction d'un réseau hydrographique contigu; et (2) l'utilisation de nouvelles étapes d'étalonnage pour surmonter les limites de la variabilité spatiale de l'étendue du modèle et du nombre de sites d'étalonna...

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