A study of clay minerals in bottom sediments of the James River estuary, Virginia, was performed to determine the predominant factors influencing their distribution. Analyses of 151 samples indicate that the factors of differential settling, flocculation, and diagenesis have minor or no effects, whereas estuarine circulation exerts the dominant influence on the clay mineral distribution. Two characteristic clay suites are present in the James River estuary; the James River clay suite is kaolinite-illite-dioctahedral vermiculite, and the Chesapeake Bay entrance bears an illite-chlorite montmorillonite suite. Mixing between the two suites occurs as a result of the estuarine circulation dynamics, which cause upstream transport of marine sediments. The upstream limit of mixing is located in the region where the surface of no net motion intersects the river bottom. Mutual dilution of the two suites by estuarine mixing is the predominant factor governing the clay mineral distribution of all clay minerals identified.