Overbank and channelfill deposits of the modern Yellow River delta

Abstract

The Huanghe is noted for its high transport rate of silt and clay, which may reach depth-averaged values of 200 kg m −3 during peak discharge. The sediment load transported through the river on entering the delta plain, amounts to 10 12 kg per year. In contrast to most other large deltas only one distributary channel is active at any one time. The high sediment load causes the rivermouth to prograde at a yearly rate of 1–4 km into the shallow (less than 20 m deep) Bohai gulf. The vertical aggradation of the channel belt and mouth bar complex is also rapid (decimetres per year on average), so that after a normal average of twelve years increasing channel instability and avulsion create the start of a new delta lobe. A series of satellite images covering the last fifteen years has provided insight in the evolution of the river pattern as well as the progradation of the delta front. A newly developed distributary passes from a multichannel to a single, straight channel system, and ends with the formation of meanders. The protruding mature delta lobe shows a radiating pattern of crevasse channels. Overbank/ crevasse deposits are made of vertically stacked dm-scale waning flow sequences, structurally characterized by (from bottom to top) small scour-and-fills, even (parallel) lamination, and climbing-ripple crosslamination. Accumulation rates on crevasse splays can be predicted on the basis of estimated river sediment discharge. It can be concluded that each sequence has been deposited within a few hours, and that tidal waterlevel fluctuations may have played a role in the generation of a single sequence.