The relative amounts of montmorillonite, illite, and chlorite-kaolinite in the bottom sediments vary considerably between the reservoirs of the Columbia River. Discriminant function analyses indicate significant differences in three areas of the Columbia River based on the variability of clay minerals in six groups of samples from the reservoirs. Samples from tributaries of the lower Columbia differ significantly from all of the other groups. The presence and distribution of two weathering environments within the Columbia basin may account for some of the agreement between observed clay fractions and those predicted from provenance evaluations and may confirm the reliability of discriminant analysis in distinguishing between two groups on the basis of clay variability. The use of discriminant function analysis is potentially valuable for vertical as well as horizontal correlation.