Response time of global deltas to changes in fluvial sediment supply

Abstract

Fluvial sediment supply supports global delta growth, yet the relationship between these two key variables remains difficult to verify for modern river deltas. Part of the complication might be that deltas are slow to adjust to changes in fluvial sediment supply. To test this hypothesis, here we analyze yearly time series of fluvial sediment supply and delta land area changes for 60 major deltas through 1990-2020. Cross-correlations show that, globally averaged, changes in delta growth rate lag 6 years behind fluctuations in fluvial sediment supply. For the 24 deltas with increased sediment supply, the lag time of land change rates was reduced to 1 year; while the 36 deltas supplied with lower fluvial sediment loads lagged by 6 years. High sediment availability in river-dominated deltas leads to a quicker response of delta land area to upstream supply changes, and deltas with stronger tidal dominance have a subdued response with long lag times. Our findings here highlight the high vulnerability of deltas and their ecosystems to changing external drivers.

Fig. 1. Global distribution of major river deltas and changes in fluvial sediment load and delta land area.

a The location of the 60 studied major river deltas, shows the annual fluvial sediment loads entering the delta from the basins and the corresponding change proportions between 1990 and 2020. b Numbers of rivers with different proportions of sediment load variation. c, d Time series and linear fitting trends of annual total fluvial sediment load and active delta land area during the last 31 years.

Fig. 2. Schematic illustration of delta land area changes and the cross-correlation analysis.

a Schematized delta land gain and land loss. b A conceptual diagram showing the fluvial sediment load and delta land change rate with illustrated change at t1 and t2, respectively, and their lag time. c Calculated cross-correlation coefficients of different lag times and the primary lag time corresponded to the position of the highest correlation coefficient.

Fig. 3. Cross-correlation analyses of fluvial sediment load and delta land area change rates.

a Time series of total annual fluvial sediment load and delta land area change rate in all deltas during 1990–2020. b Cross-correlation coefficients of different lag times in 60 deltas with a primary lag time of 6 years. c Linear correlation between the lagged total sediment load and land area change rate in 60 deltas. d–f The time series of annual total fluvial sediment load and land area change rate in 36 deltas with decreased sediment supply, shows cross-correlation analyses with a primary lag time of 1 year, the linear fitting of lagged data is also significant. g–i Time series of total fluvial sediment load and land area change rate in 24 deltas with increased fluvial sediment input, shows cross-correlation analyses with a lag time of 6 years and the linear fitting of lagged data.

Fig. 4. Delta-scale lag response timescales.

a The global distribution of the 23 classified deltas, shows the annual fluvial sediment load (Q_river) and the studied lag time between 1990 and 2020. b A ternary diagram of 23 deltas with different relative river, tidal, and wave forcings, the corresponding color of the solid circle represents lag time in this study.

Fig. 5. Lag times and the relative fluvial-marine forcings in deltas.

a The relationship between the T-value (the ratio of river and tidal forcings) and lag time in 16 river-/tide-dominated river deltas, shows an overall significant positive linear fitting trend. b The relationship between the entire delta area and lag time in 16 river-/tide-dominated river deltas indicates that a larger delta responds faster to changes in annual fluvial sediment load between 1990 and 2020.