Floodplain evolution in a small, tectonically active basin of northern California



Sediment cores were collected along floodplains in the Navarro River basin of coastal northern California to examine the controls on floodplain evolution in a tectonically active setting. Sedimentary strata were subsampled for organic content, bulk density, and grain size measurements. Organic samples were analysed for 14C age, which yielded net-averaged sedimentation rates for all cores. Overbank deposition rates decreased at all study sites through time and declined in the downstream direction. The ability of intermediary-order streams to store sediment in floodplains decreased the ability of highest-order streams to record sediment-pulse events. The effects of anthropogenic disturbance, primarily logging, on long-term overbank deposition rates were minimal. Climatic variability, by affecting sediment loading in the channel network, is the principal control on floodplain evolution through the Holocene. A hypothetical model is proposed to explain overbank deposition rates in the Navarro basin, which may be extrapolated to the northern-coastal California region during the late Pleistocene and Holocene. The complexities observed in sediment storage and routing in this study imply that caution should be made when extrapolating sediment-yield measurements obtained at river mouths or coastal shelves to geomorphic events within small, tectonically active basins. Copyright © 2003 John Wiley & Sons, Ltd.