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Sr87/Sr86 values of Columbia River bottom sediments vary consistently with the age and compositional nature of provenance as indicated by previous mineralogic studies. These values range from 0.717 at Grand Coulee Reservoir where sediments are derived in part from a terrane of older metamorphic, plutonic, and sedimentary rocks, to 0.704 near the mouth of the Columbia where young volcanic rocks of the Cascades are the dominant source. Isotopic variation in sediments along parts of the Columbia River—Grand Coulee to McNary—can be explained by simple mixing of end-member detrital components. These results suggest that strontium isotopes may provide a useful clue in reconstruction of source terranes for graywackes in which mineralogic evidence may be destroyed or masked by diagenetic or low-grade metamorphic processes.

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