- Water quality impacts both human and ecosystem health. Many pharmaceuticals and personal care products are considered contaminants of emerging concern (CEC) due to their bioactive properties. Among these, metformin—the most commonly prescribed drug for treatment of type 2 diabetes—has been reported at high concentrations (μg L⁻¹) in stream, lake, and estuary surface waters in the United States. However, a propensity for broad-scale CEC surveys has prevented seasonal and spatial monitoring of metformin in river systems. Moreover, the effect of metformin in river food webs remains poorly explored, despite its known action on conserved eukaryotic enzymes (AMPK/SnRK1/SNF1). This project comprised the first spatiotemporal characterization of metformin in a high-discharge river system and examined the effects of metformin on aquatic primary producers. Monthly water samples were taken at eight year-round sites and five additional summer-sites in the lower Columbia River from October 2016-September 2017. Metformin and its breakdown product, guanylurea, were determined from water samples using direct injection with liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). Laboratory toxicity assays using PAM fluorometry and environmentally or sewage relevant doses (50-500 μg L⁻¹) probed for effects of metformin on photosynthetic efficiency in green algae (Chlorella vulgaris) and diatom (Thalassiosira weissflogii) cultures. Metformin and guanylurea were successfully detected and quantified in the lower Columbia River (METAVG = 67.4 ng L⁻¹, GUANAVG = 32.6 ng L⁻¹). High variation in metformin concentrations was only partially explained by seasonal river discharge and riverine sources; but possible sorption effects suggested additional physicochemical influence. Metformin effects on algae depended on exposure time, dose, and species, with reduced photosynthetic efficiency of C. vulgaris cultures within 24 h of high dose exposure (500 μg L⁻¹), and within 72 h of sewage effluent-equivalent exposure (50 μg L⁻¹). Overall, these results are consistent with the expected behavior of a polar basic cation species in a high-flow river system and a SnRK1-activator in algae. This study illustrates the importance of investigating individual CECs in water systems in order to elucidate fine-scale processes governing distributions, behavior, and, ultimately, environmental risk.