February BRGS Meeting: Holocene, Modern and Future Sediment Transport Through the Mouths of the Pearl and Indus Rivers

Holocene, Modern and Future Sediment Transport Through the Mouths of the Pearl and Indus Rivers

Peter D. Clift

Department of Geology & Geophysics, Louisiana State University

Abstract

The Pearl River of southern China and the Indus River in SW Asia, with its delta in Pakistan are both large monsoon-fed rivers that are home to significant human populations. Sealevel and monsoon controlled discharge have controlled the flux of sediment through the river mouths in the last several thousand years. Zircon U-Pb dating shows that sandy sediment in the Indus travels slowly and is mostly deposited near the river mouth during the Holocene, with limited supply to offshore shelf clinoforms and almost nothing to deeper waters. Deep-water sand sedimentation only occurs during sea-level low-stands. In contrast, silt and clay have more readily been transported into the submarine canyon, as well as the clinoforms and this transport is largely unbuffered from the river mouth. The silt is however strongly advected to the east by long-shore currents. Smectite clay is preferentially deposited in the river mouth rather than transported far offshore relative to other clays, although the proportion of smectite is also seen to increase after 5 ka, probably because of reworking of smectite-rich soils in the flood plains driven by the establishment of agriculture in this area around that time. Damming of the Indus during the 20^thCentury has dramatically reduced the flow of water and sediment to the river mouth and resulting in salinization of the farm lands and retreat of the coast due to subsidence and sea-level rise.

            In contrast, the Pearl River presently carries little sand but its dominantly muddy load is not delivered into deep water because of the modest discharge and very wide continental shelf, combined with a strong west-flowing coastal current. The onshore delta is heavily settled and developed compared with the Indus and is vulnerable to future sealevel rise. Like many deltas it has advanced seawards since the stabilization of sealevel at ~6 ka. The source of sediment has been mostly stable through time but also shows the impact of the start of agriculture, at around 2.7 ka in this region, and mature, weathered, soil material was reworked into the river. Prior to this time enhanced recycling from the flood plains is associated with times of strong monsoon rains. Again, damming has sharply reduced sediment flux to the ocean over the past 40 years from >100 Mt/yr to ~20 Mt/yr. Nonetheless, sedimentation in the coastal zone is seen as a carbon-neutral method for protecting the coasts and their populations from rising sea levels but would require management of the river basin. A combination of offshore artificial reefs to reduce wave-recycling and mangrove plantations to capture sediment near the coast is a potentially effective combination to secure the region’s future but would require increasing sediment supply and the redirecting of sediment to the northeastern distributaries and away from those that are directing sediment more directly out into the South China Sea. Preserving the sediment within the bay that is formed between the delta front, and the coastline to the east is critical for the long-term stability of the area.

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