Clay Minerals of the Columbia River: A Qualitative, Quantitative, and Statistical Evaluation

Abstract

ABSTRACT The relative amounts of montmorillonite, illite, and chlorite-kaolinite in the bottom sediments vary considerably between the reservoirs of the Columbia River. The relative fractions of montmorillonite increase progressively downstream; illite shows an inverse trend; the amounts of chlorite-kaolinite remain consistently low throughout the 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 reservoirs: Grand Coulee (group 6) in the upper reaches, Rocky Reach, Wanapum, and Priest Rapids (group 5) in the mid-reaches, and Bonneville, Dalles, McNary, and Ice Harbor (groups 1-4) in the lower reaches of the river. Samples from tributaries of the lower Columbia River (group 7) differ significantly from all other groups. The presence and distribution of two distinct types of weathering environments within the Columbia River Basin may account for some of the agreement between observed clay mineral fractions and those predicted from source area evaluations, and they may confirm the reliability and sensitivity of the discriminant analysis in distinguishing between two groups on the basis of clay mineral variability. The use of the discriminant function analysis in clay mineral studies is potentially valuable for vertical as well as horizontal correlation.