Modeling the growth limit of seven major Holocene river deltas in Asia

River deltas grow in response to sediment discharge from the land, and there is a limit of growth. A conceptual geometric model is used in this paper, based on the principle of mass conservation, to investigate the impact of the significant decrease in sediment supply to the growth of deltas taking seven large Asian river deltas as examples. Furthermore, the sediment retention index for the Holocene period, the critical river input for maintenance of a delta and the growth limit without human interferences are discussed. Preliminary analyses of these deltas indicate that the area of delta plain is roughly in an order of 10³~10⁴km², and the volumes of 10¹~10³km³, and both are significantly positively correlated with the fluvial flux before the large numbers of dams being constructed. The range of the Holocene retention index varies between 36%~54%, with an average of 45%, indicating that more than half of the sediment escaped offshore and supported the deposition of shelf mud or deep sea fan. For the Yangtze, the Pearl, the Red, the Mekong and the Indus rivers, their sediment discharges are now below the level required to maintain the area of the sub-aerial deltaic plain and the volume of the entire deltaic deposits, except for the Irrawaddy and the Ganges-Brahmaputra rivers, of which the river discharges are higher than the critical level required to maintain their total volumes, but lower than the critical level required to maintain deltaic plain areas. Without coastal protection projects, the area and volume for the former five deltas would drop sharply, but increase for the latter two. The present study has established the interrelationships between the growth magnitude, Holocene sediment retention index, the critical fluvial sediment flux and the growth limit of the seven Asian major river deltas, demonstrating that the method based on the conceptual geometric model has a great potential to predict the long-term evolution of river deltas.