Geomorphic Dynamics and Environmental History of a Floodplain Tract

A multi-proxy approach was used to examine the geomorphic dynamics and environmental history of an upper deltaic floodplain tract in the Sacramento–San Joaquin Delta, California. Three long cores were collected from the McCormack– Williamson Tract (MWT) and these cores were analyzed for bulk density, loss-on-ignition, fine (clay and silt) content, Al concentration, magnetic susceptibility, pollen, and charcoal. Radiocarbon dates obtained for the cores were converted into calendar years and an age–depth model was constructed. Long-term vertical accretion and sedimentation rates were estimated from the age–depth model. Cross-core relations show that coarse sediment generally accumulates more rapidly and has greater magnetic susceptibility compared to fine sediment. Percentage fine and LOI data show a strong linear relationship that indicates flooding is the primary mechanism for the deposition of particular organic matter on the floodplain and that landscape wash load has contributed a highly consistent fraction of persistent organic matter averaging 5·5 per cent to the site. Down-core grain size profiles show two hydrological domains in the cores, namely millennial fine–coarse fluctuations superimposed on general up-core fining. Coarse sediment is viewed as channel or near-channel overbank deposits, whereas fine deposits are considered to be distal overbank flood deposits. The coarse–fine fluctuations are indicative of changing depositional settings as channels migrated laterally across MWT, whereas the upward fining trend reflects a combination of self-limiting overbank deposition as floodplain elevation increased and decreasing competence as sea-level rise reduced flood-pulse energy slopes. MWT has been cross-cut and incised numerous times in the past, only to have the channels abandoned and subsequently filled by fine sediment. The channels around MWT attained their modern configuration about 4000 years ago. MWT likely came under tidal influence at about 2500 cal BP. Wetlands have recently developed on MWT, but they are inorganic sediment dominated. Copyright © 2004 John Wiley & Sons, Ltd.