AbstractThe mineralogy and related chemistry of selected lake and stream sediments from the upper Qu'Appelle River basin in southern Saskatchewan, Canada, was examined. The principal crystalline clay minerals present in the colloidal fractions of the sediments are smectite, vermiculite, kaolinite, mica, quartz, and feldspar. The cation‐exchange capacity of the sediments varies from 16.3 to 28.8 meq/100 g, and the pH ranges from slightly acid to mildly alkaline. The hydrous oxides of Al, Fe, Mn, and Si are present in significant quantities in both the colloidal and noncolloidal fractions of the sediments; this is significant because these oxides likely exert the dominant chemical control on the movement of many nutrients and pollutants in aquatic environments. Calcium and Mg carbonates comprise a significant fraction of the sediments. Most of the C in the sediments is in the organic form; the organic C content varies from 1.2 to 6.0%. Because crystalline clay minerals, poorly ordered sesquioxides, and organic matter play a vital role in the retention and cycling of nutrients and pollutants in aquatic environments, a knowledge of the nature and properties of sediments is fundamental to efforts to preserve and improve the quality of natural water system