<p>Nitrogen (N) and phosphorus (P) concentrations in many northern prairie rivers have been increasing due to anthropogenic activities. While long-term trends in total N and P have been well documented, there remains limited knowledge regarding trends in dissolved fractions as well as the associated effects of shifting nutrient loadings on nutrient stoichiometry (i.e., N:P) of river water. We assessed long-term (25-year) trends in total and dissolved N and P concentrations and N:P at 11 monitoring stations situated on five rivers within the Red-Assiniboine River Basin in North America. We found that N and P concentrations and stoichiometry were changing through time at a majority of stations. Spatial patterns of trends were variable with no consistent directional changes in either nutrient concentrations or stoichiometry among stations, suggesting the importance of localized nutrient sources, such as wastewater treatment plants. Changes associated with catchment characteristics were the primary contributors to observed trends in nutrient concentrations and stoichiometry, whereas alterations in the streamflow regime played a comparatively minor role. Variations in the relative quantities of nutrients in the basin’s rivers may be influencing the potential for nutrient depletion, with some rivers undergoing stoichiometric shifts in the depleted nutrient. Consequently, nutrient management may need to occur at the sub-basin scale to mitigate point source nutrient pollution and protect riverine water quality throughout the basin.</p>