Abstract

Grade, a fundamental concept in river geology and geomorphology, refers to a long-term sediment balance that is accompanied by zero net deposition and erosion. Recent physical and theoretical modeling proposed the notion that downstream alluvial rivers can autogenically attain grade only in a particular set of environmental conditions that include a constant fall of relative sea level. We here make the first successful identification of an autogenic grade system in the geological record: the late Holocene Mekong River delta, Vietnam. From 3.5 ka to subrecent, the record of the delta exhibits peculiar features, including (1) no trace of significant sediment accumulation and erosion on the delta plain surface, (2) a delta plain surface with the same slope as the underlying shelf surface, (3) distributary channels that are stabilized in transverse directions but extend linearly basinward, and (4) a delta set thickness that matches a theoretical value. These features in combination are indicative of autogenic grade. Coastal dispersal of river-derived sediment by tides, waves, and ocean currents, as well as tectonic features and mangrove vegetation, may have contributed to the attainment and maintenance of grade. Ongoing drastic changes in sea level and human activities have caused the downstream Mekong River to become ungraded and unstable with a much higher risk of channel avulsion and shifting than in the past.

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