Sediment retention at the Changjiang sub-aqueous delta over a 57 year period, in response to catchment changes

The growth of a river delta system is controlled mainly by fluvial sediment discharge and sediment retention in the deltaic areas. In the present study, we attempt to define a sediment retention index, R, and its relation to the deposition rate of the delta. Based upon two assumptions that the amount of the sediment that escapes from the deltaic areas, Q_E, is stationary with minor fluctuations, and that there is a linear relationship between the deposition rate averaged over the sub-aqueous delta (D_av) and the deposition rate at accretion sites within the delta (D_R), the changes in the accretion/erosion patterns of the Changjiang sub-aqueous delta during the period of 1951-2007, in response to river sediment discharge changes, are analyzed. The results show that the sediment retention index can be related to the deposition rate of the sub-aqueous delta; the spatial-temporal distribution pattern of the deposition rate reveals the behaviour of sediment retention of a delta system. For the Changjiang sub-aqueous delta, fluvial sediment discharge data, together with Pb-210 based deposition rates, provide useful information on sediment retention. Changes in the sediment retention index and the accretion/erosion patterns of the sub-aqueous delta have taken place in response to river input changes. In order to improve our understanding of the processes associated with estuarine sediment retention, sediment cores with sufficient spatial coverage may be collected and analyzed to establish accurate Q_E-R and D_av-D_R relationships. In combination with numerical modeling of sediment transport, these relationships form a basis for the analysis of sediment retention mechanisms.