Characterization of both the source materials and the bottom sediments of an estuary by magnetic characters, on a particle size basis, allows the source of the bottom sediments to be determined and indicates a shift in sediment sources, corresponding to changing land-use patterns. The major source of the sediments in the lower part of cores from the middle of the estuary is eroding shorelines. Soil-derived particulates, from the uplands, dominate the upper part of the cores. This change in source occurred in the early part of the 19th century. The magnetic character of the most recent sediments suggests a progressive depletion, through time, of the upland soils in the finest fractions.