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Global-scale human impact on delta morphology has led to net land area gain

Abstract

River deltas rank among the most economically and ecologically valuable environments on Earth. Even in the absence of sea-level rise, deltas are increasingly vulnerable to coastal hazards as declining sediment supply and climate change alter their sediment budget, affecting delta morphology and possibly leading to erosion1,2,3. However, the relationship between deltaic sediment budgets, oceanographic forces of waves and tides, and delta morphology has remained poorly quantified. Here we show how the morphology of about 11,000 coastal deltas worldwide, ranging from small bayhead deltas to mega-deltas, has been affected by river damming and deforestation. We introduce a model that shows that present-day delta morphology varies across a continuum between wave (about 80 per cent), tide (around 10 per cent) and river (about 10 per cent) dominance, but that most large deltas are tide- and river-dominated. Over the past 30 years, despite sea-level rise, deltas globally have experienced a net land gain of 54 ± 12 square kilometres per year (2 standard deviations), with the largest 1 per cent of deltas being responsible for 30 per cent of all net land area gains. Humans are a considerable driver of these net land gains—25 per cent of delta growth can be attributed to deforestation-induced increases in fluvial sediment supply. Yet for nearly 1,000 deltas, river damming4 has resulted in a severe (more than 50 per cent) reduction in anthropogenic sediment flux, forcing a collective loss of 12 ± 3.5 square kilometres per year (2 standard deviations) of deltaic land. Not all deltas lose land in response to river damming: deltas transitioning towards tide dominance are currently gaining land, probably through channel infilling. With expected accelerated sea-level rise5, however, recent land gains are unlikely to be sustained throughout the twenty-first century. Understanding the redistribution of sediments by waves and tides will be critical for successfully predicting human-driven change to deltas, both locally and globally.

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Fig. 1: Global distribution of predicted pristine delta morphologies.
Fig. 2: Predicted delta morphologic change from pristine to future equilibrium conditions.
Fig. 3: Rates and drivers of delta land area change over the period 1985–2015.

Data availability

All primary sources (OSU TOPEX50, NOAA WaveWatch47, USGS HydroSheds36, USGS SRTM37, WBMSed42 and AquaMonitor20 data) are publicly available. Wave and tide data can also be found at https://jhnienhuis.users.earthengine.app. The resulting morphological predictions for all 10,484 deltas are available as .mat and .kml files at https://doi.org/10.17605/OSF.IO/S28QB. Source data for Figs. 13 are provided with the paper.

Code availability

The Matlab computer code that reproduces our findings is available at https://github.com/jhnienhuis/GlobalDeltaChange and https://osf.io/s28qb/.

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Acknowledgements

This research was supported by US National Science Foundation award EAR-1810855, Netherlands Organisation for Scientific Research (NWO) vi.veni.192.123 and a scholarship from the Wageningen University Postdoc Talent Program, all to J.H.N. J.C.R.’s efforts were supported by the DOE BER Regional & Global Climate Modeling Program through the HiLAT project. D.A.E. was supported by National Science Foundation awards 1812019 and 1426997. A.J.F.H. was funded by the NWO within Vici project ‘Deltas out of shape: regime changes of sediment dynamics in tide-influenced deltas’ (grant NWO-TTW 17062). We thank P.J.J.F. Torfs (Wageningen University and Research) for help with the adopted statistical methodology.

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Contributions

J.H.N., A.D.A. and D.A.E. conceived the study. A.J.K. assisted with the global sediment flux calculations. J.H.N. carried out the study and wrote the initial draft. J.H.N., A.J.F.H. and T.E.T. discussed the results. All authors contributed to the writing of the manuscript.

Corresponding author

Correspondence to J. H. Nienhuis.

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The authors declare no competing interests.

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Peer review information Nature thanks Nick van de Giesen and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Overview of the algorithm that identifies river deltas using HydroSheds data.

a, HydroSheds drainage basins and the included deltas are shown for Veracruz, Mexico. b, Close-up of a, showing the included deltas and the tracked river channel for the channel slope calculation. Scale bars show the resolution of the WaveWatch47 and TOPEX datasets50.

Extended Data Fig. 2 WBMSed model predictions.

a, Discharge per cell. b, Sediment yield42.

Extended Data Fig. 3 WBMsed model predictions of human-induced change to the deltaic fluvial sediment flux.

Colours indicate the ratio of the modern fluvial sediment flux (\({Q}_{{\rm{river}}}^{{\rm{d}}}\); here Qriver,dist) to the flux in a world without anthropogenic modifications42 (\({Q}_{{\rm{river}}}^{{\rm{p}}}\); here Qriver,prist).

Extended Data Fig. 4 Characterization of data used for wave- and tide-driven deltaic sediment flux.

a, Global maximum potential alongshore sediment transport (Qwave) based on the WaveWatch 30-year hindcast data47. b, Global estimate of mean tidal amplitude based on the OSU TOPEX data50.

Extended Data Fig. 5 Example of recent deltaic land area change for the north shore of Java, Indonesia.

Land loss and land gain were measured using Landsat (http://landsat.usgs.gov/) images from Google Earth Engine52 based on the Deltares Aqua Monitor35. Here, deltas have expanded recently because of human-induced increases in the fluvial sediment flux. The top image shows the coastal change, with the red markers and black outlines representing individual deltas and their coastlines, respectively.

Extended Data Table 1 Confusion matrix of the number of deltas on Madagascar
Extended Data Table 2 Confusion matrix of the delta morphologic prediction based on a validation dataset of 312 deltas
Extended Data Table 3 Yearly deltaic land gain, loss and net gain for different regions
Extended Data Table 4 Predicted sediment transport fluxes for a selection of well-known deltas
Extended Data Table 5 Comparison of net land gain estimates with case studies from the literature

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Nienhuis, J.H., Ashton, A.D., Edmonds, D.A. et al. Global-scale human impact on delta morphology has led to net land area gain. Nature 577, 514–518 (2020). https://doi.org/10.1038/s41586-019-1905-9

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