Global River Sediment Retention on Delta Plains

Abstract

Fluvial sediments have helped form approximately 830,000 km² of coastal deltaic land, a large part of which since the Holocene slowdown of global sea-level rise. The rate at which deltas have retained fluvial sediment to build deltaic land, however, has varied greatly between different deltas. Sources of variability as well as global estimates of sediment retention remain poorly resolved, limiting the assessment of future delta change. Here, we compare delta area and thickness against fluvial sediment supply to derive estimates of delta plain sediment retention for ∼1,700 deltas, representing 93% of the global fluvial sediment supply. We find that 28% ± 14% (standard error of the mean) of the global fluvial sediment discharge is retained on delta plains, totaling 5.8 Gt per year, and equating to roughly 4.3 mm yr⁻¹ as a global average sedimentation rate. The average delta, however, retains much less, about 4% ± 1%, because many smaller deltas do not retain much sediment. We observe a large difference between active and passive margins, with sediment retention much higher along the latter. Tide-dominated deltas have the most efficient morphology, and retain 11% ± 4% on average but up to 70% when weighted by sediment flux. This is much higher than the 7% ± 2% and 2% ± 1% retained by river- and wave-dominated deltas, respectively. As larger deltas retain a greater proportion of their supply, this study suggests a positive feedback that enables larger deltas to grow faster than smaller ones.