GLOBAL CONTROLS ON RIVER DELTA SEDIMENT RETENTION

Fluvial sediments have collectively formed about 900,000 km² of deltaic land since Holocene sea-level rise slowed down. The rate at which deltas have retained fluvial sediment to form deltaic land, however, varies greatly between different deltas. Here we investigate and quantify sediment retention for deltas globally. We define delta sediment retention as fraction of the delivered fluvial sediment (m³) retained within the delta topset and foreset (m³).

Based on WBMSed fluvial suspended sediment flux data and NASA landsat imagery of land area change over the past 30 years, we find that deltas, on average, retain 40% of their fluvial sediment. Many deltas have lost more volume than can be explained from an increase in accommodation space, despite fluvial sediment import. Effective sediment retention for those deltas is therefore below 0%. Half of all river deltas retain less than 5%.

Delta morphology exerts significant control on sediment retention. Tide dominated deltas retain 50% on average. Wave and river-dominated deltas are less efficient and retain about 40%. Although larger deltas retain less sediment, the inefficiency of river-dominated deltas cannot be explained by their size: tide dominated deltas have greater sediment loads, on average. Instead, river mouth processes including river plume characteristics likely play an important role in deltaic sediment retention.