Small estuaries of the northwestern United States are fed by high-gradient streams which range widely in mean fluvial-discharge rates. Sediment composition in these estuaries differs with grain size and with the relative abundances of river- and beach-derived sand. In order to determine to what degree sediment composition is related to the relative influence of tidal and river flow in these active-margin estuaries, the modern sediment compositions in six Pacific Northwest estuaries were analyzed by standard grain-size and heavy-mineral techniques. The average textural and sand-source compositions for each bay were calculated on a percent surface-area basis. Estuarine sediment compositions are compared and found to be correlated with a hydrographic parameter H R (mean tidal-prism volume: mean fluvial discharge rate x 6 hours) computed for each bay. Both % mud and % beach sand increase as the dimensionless hydrographic ratio (H R ) increases. Unusual results in two of the estuaries are attributed to man-made alterations in one estuary and to eolian transport of beach sand into the other estuary.