The mineral and chemical composition of sediments deposited in the three lowermost reservoirs of the Columbia River is remarkably similar to the composition of many graywackes. Lithic fragments are abundant. In comparison with an "average" sandstone, the sediments have low concentrations of silica and high concentrations of all other major constituents, except calcium. Sodium is more abundant than potassium. The sediments are generally better sorted than graywackes. If graywacke texture is post-depositional in origin, Columbia River-type sediments could be expected to form graywackes upon deep burial without any significant addition or removal of material.