Sustained increase in suspended sediments near global river deltas over the past two decades

Abstract

River sediments play a critical role in sustaining deltaic wetlands. Therefore, concerns are raised about wetlands' fate due to the decline of river sediment supply to many deltas. However, the dynamics and drivers of suspended sediment near deltaic coasts are not comprehensively assessed, and its response to river sediment supply changes remains unclear. Here we examine patterns of coastal suspended sediment concentration (SSC) and river sediment plume area (RPA) for 349 deltas worldwide using satellite images from 2000 to 2020. We find a global increase in SSC and RPA, averaging +0.46% and +0.48% yr^-1, respectively, with over 59.0% of deltas exhibiting an increase in both SSC and RPA. SSC and RPA increases are prevalent across all continents, except for Asia. The relationship between river sediment supply and coastal SSCs varies between deltas, with as much as 45.2% of the deltas showing opposing trends between river sediments and coastal SSCs. This is likely because of the impacts of tides, waves, salinity, and delta morphology. Our observed increase in SSCs near river delta paints a rare promising picture for wetland resilience against sea-level rise, yet whether this increase will persist remains uncertain.

Fig. 1. Global pattern of coastal suspended sediment concentration (SSC) between 2000 and 2020.

a The long-term mean SSC (unit: mg/L) for 349 deltas. The different circle sizes represent varying magnitudes of water discharge, while the different circle colors indicate different levels of SSC. b–d Box plots of long-term mean SSC in different continents (AF:Africa, AS:Asia, EU:Europe, NA:North America, OC:Oceania, SA:South America) (b), various delta morphologies (c), and the Arctic (with latitude >50°N) and non-Arctic deltas (d). The box plots in b–d show the distributions (10, 25, 50, 75, and 90% values) of SSC.

Fig. 2. Trends of coastal suspended sediment concentration (SSC) and river plume area (RPA) between 2000 and 2020.

a Spatial patterns of the SSC trends (Mann–Kendall test) in 349 deltas. The latitudinal profiles exhibit the percentages of deltas with significant (p < 0.05) and insignificant SSC change trends (increase or decrease). b Variations in monthly mean SSC and RPA, along with their long-term change trends at global scales. The orange and blue dash lines represent the long-term trends of SSC and RPA, respectively. These trends were derived based on long-term monthly anomaly SSC and RPA, which were estimated as the difference between the monthly mean SSC and RPA and their long-term average for that month (see Methods). The Sen’s slope and p value obtained from the Mann–Kendall test are annotated.

Fig. 3. Change trends of Q_river and suspended sediment concentration (SSC) between 2000 and 2020 for 186 deltas.

a Spatial patterns of the trends (increase or decrease) in Q_river and SSC. The different colors represent different combinations of Q_river and SSC trends (Inc.: increase; Dec.: decrease). b Scatterplot of change rates (Sen’s slopes from the Mann–Kendall test) in Q_river and SSC for 186 deltas. Each quadrant (Q1, Q2, Q3, and Q4) represents a combination of SSC and Q_river trends. The numbers in parentheses represent the percentages of deltas. The circle, triangle, and square represent river-, tide-, and wave-dominated deltas, and different colors represent different continents. The circles filled with orange represent that both the change rates of SSC and Q_river are magnified 10 times, respectively. The circles filled with blue and green indicate a magnification of 10 times separately in only the change rates of SSC or Q_river, and the circles filled with red indicate a tenfold decrease in Q_river. The gray lines point out several representative deltas. c–f Box plots of long-term mean Q_river, Q_tide, Q_wave, and salinity in different quadrants in b. The box plot shows the distributions of 10%, 25%, 50%, 75%, and 90% values.

Fig. 4. Relationships between long-term mean suspended sediment concentration (SSC), Q_river, Q_tide, Q_wave, and salinity.

a–d The relationships between SSC and Q_river (a), Q_tide (b), Q_wave (c), and salinity (d). The number of deltas (n), correlation coefficients (r), and p values were annotated. Out of our 349 deltas, the Q_river is only available for 186 deltas and salinity is available for 139 deltas, thereby the relationship analyses between Q_river, Q_tide, Q_wave, salinity, and SSC were conducted for these deltas. The correlation coefficients are obtained based on the logarithm-transformed SSC and different factors. e Contributions of four drivers (Q_river, Q_tide, Q_wave, and salinity) on long-term SSC changes in 139 river deltas. The contributions (see Supplementary Table 3) of four drivers in each delta are presented through pie charts. The pie charts consist of five colors, representing the four drivers and residuals, with the size of each slice indicating the respective proportion.