# Concerns about data linking delta land gain to human action

Florin Zăinescu, Edward J. Anthony, Alfred Vespremeanu-Stroe, Manon

Besset, Florin Tătui

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Florin Zăinescu, Edward J. Anthony, Alfred Vespremeanu-Stroe, Manon Besset, Florin Tătui. Concerns about data linking delta land gain to human action. Nature, 2023, 614 (7947), pp.E20-E28. 10.1038/s41586-022-05624-x. insu-04131493

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# Concerns about data linking delta land gain to human action

https://doi.org/10.1038/s41586-022-05624-x

Received: 5 January 2021

Accepted: 2 December 2022

Published online: 8 February 2023

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Florin Zăinescu<fnref n="1" />,²✉, Edward Anthony<fnref n="3" />, Alfred Vespremeanu-Stroe²,⁴, Manon Besset⁵ & Florin Tătui²

ARISING FROM J. H. Nienhuis et al. Nature https://doi.org/10.1038/s41586-019-1905-9 (2020)

River deltas are diverse, economically and ecologically important eco-
systems that are increasingly vulnerable to environmental change.
A recent study reported that human impacts on the morphology of
deltas at the global scale has led to a net gain in land area<fnref n="1" />. However,
we suggest that an unconventional definition of deltas, miscellane-
ous coastal features unduly characterized as deltas and misidentified
changes in delta area have led to inaccurate statistical analyses in the
study<fnref n="1" /> and that a recently published correction² does not address our
concerns. We call for the rigorous verification and analysis of large
environmental datasets.

Nienhuis et al.<fnref n="1" /> define marine river mouths with parameters above the thresholds for fluvial water (more than 1 m<fnref n="3" /> s⁻<fnref n="1" />) and for sediment discharge (more than 0.01 kg s⁻<fnref n="1" />) as deltas. This definition is different from standard geoscientific definitions for deltas<fnref n="3" />⁻⁶, but nevertheless uses morphological terms that are specific to deltas<fnref n="3" />,⁴ that are distinct from estuaries, such as shoreline protrusion and a triangular shape.

The simplified assumptions for the redistribution of sediment flux at river mouths by rivers, waves and tides used by Nienhuis et al.<fnref n="1" /> do not include the sediment-dispersal role of these agents that render most river mouths devoid of deltas<fnref n="3" />,⁵,⁶. These assumptions also do not include sedimentation pathways that interlink the connected upbuilding and outbuilding components of deltas (the subaerial delta plain, subaqueous delta front and pro-delta) that differentiate them from simple estuaries.

Nienhuis et al.<fnref n="1" /> use automatically generated ‘buffer’ areas to iden-
tify land change from global spatial data⁷, and propose the exclusion
of areas distant from channel banks and shorelines. We find that this
automatic identification of buffers, without verification of their accu-
racy, leads to thousands of features that were wrongly identified as
deltas by Nienhuis et al.<fnref n="1" />, including estuaries, built-up areas, rocky
coasts (Fig. 1), beach-ridge plains and multiple distributary mouths of
individual deltas. Unfortunately, considering such a diverse collection

<figure><img src="image_3.png" /><figcaption>**Fig. 1 | Previous categorization of river mouths as deltas.** Nienhuis et al.<fnref n="1" /> characterized 270 deltas in the British Isles (blue dots, left), none of which are referred to as deltas in scientific bibliographic databases. Map data © 2021 GeoBasis-DE/BKG (©2009), Google. **a–h**, Examples of misidentified deltas.</figcaption></figure>

The grey zones show the area of each 'delta' used to calculate the natural delta land change by Nienhuis et al.<fnref n="1" />, but these zones wrongly overlie rocky coasts (a), beaches (g), rias (d), barriers (c), estuaries (f, h) and cities (b, e). Scale bar, 100 km.

<footnote n="1">¹Institut Terre Environnement Strasbourg, UMR 7063, CNRS, Strasbourg, France. ²GEODAR Research Center, Faculty of Geography, University of Bucharest/RYA, Bucharest, Romania.</footnote>

<footnote n="3">³Aix Marseille University, CNRS, IRD, INRAE, Coll France, CEREGE, Aix-en-Provence, France. ⁴Sfântu Gheorghe Marine and Fluvial Research Station, University of Bucharest, Sfântu Gheorghe, Romania. ⁵i-Sea SAS, Merignac, France. ⁶e-mail: zaiflorin@gmail.com</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 2 | Misidentified land change shows no correlation with change in sediment flux.** a–c, Delta land gain shows a weak correlation with the fluvial sediment flux induced by human activity, $Q_{\text{river}}^{\text{d}} - Q_{\text{river}}^{\text{p}}$ of $R^2 = 0.16$, but there is no correlation ($R^2 = 0$) when the Yellow River Delta is removed or corrected for the entire 10,848 ‘deltas’ dataset (a) and no correlation ($R^2 = 0$) for both the largest 100 ‘deltas’ (b) and the smallest 10,748 ‘deltas’ (c). $R^2$ is the coefficient of</figcaption></figure>

determination from a linear regression model. d, Aquaculture ponds (red area in the grey buffer zone) in Yellow River Delta were misidentified by Nienhuis et al.¹ as natural loss. Buffer zones should capture only natural land change, not human-transformed lands. Green areas indicate land gain. e, f, Example land-cover changes, highlighted in the boxed area in d, in 1985 (e) and the present (f). d, Map data © Google. e, f, Landsat images © Sentinel Hub.

of coastal morphologies as a simple continuum, culminating in 10,848 'river deltas' could undermine the gravity of contemporary concerns on delta vulnerability and the hundreds of millions of people that deltas host⁸⁻¹⁰. A random check reveals that only around 50% of the inventoried features may be defined as deltas (Extended Data Table 1 and Supplementary Methods). Nienhuis et al.¹ checked the accuracy of the existence of the deltas using 212 deltas in Madagascar, which is a biased sample. Instead, such a check should be based on a random selection of their large global dataset.

Nienhuis et al.¹ conclude that global-scale human impacts on river-sediment flux has led to gains in the delta land area, which, according to Nienhuis et al.¹, is illustrated by figure 3a using a ratio ($Q_{\text{river}}^{\text{d}}/Q_{\text{river}}^{\text{p}}$) of disturbed (by dams and land-use changes; $Q_{\text{river}}^{\text{d}}$ over pristine (before any substantial human influences; $Q_{\text{river}}^{\text{p}}$) fluvial sediment fluxes, regressed against land change for each delta. In correspondence with Nienhuis et al.¹, they claim that they did not use $Q_{\text{river}}^{\text{d}}/Q_{\text{river}}^{\text{p}}$ to support their conclusion but, instead, used $Q_{\text{river}}^{\text{d}} - Q_{\text{river}}^{\text{p}}$ to attribute 16% of the delta land gain to human impact. However, Nienhuis et al.¹ do not provide any evidence, graphical or otherwise, that substantiates this conclusion based on $Q_{\text{river}}^{\text{d}} - Q_{\text{river}}^{\text{p}}$. We tested $Q_{\text{river}}^{\text{d}} - Q_{\text{river}}^{\text{p}}$ against land change using their original datasets and methodology and obtained an $R^2$ of 0.16 (on which the 16% from Nienhuis et al.¹ is based). However, this value is upheld by the Yellow River Delta (Fig. 2), a single delta out of 10,848 river mouths that is simply considered, in their published Author Correction², to be an example for which their methodology may produce errors. Our analysis shows that the land area change of -8.3 km² yr⁻¹ for this delta, as described in Nienhuis et al.¹, is flawed

(Fig. 2a,d) because it includes anthropogenic land change (Fig. 2e,f). Our buffer yields 2.15 km² yr⁻¹, similar to a previously published value for the same period¹¹, which identifies this delta as one of the fastest growing on Earth. When the data for the Yellow River Delta are either corrected for true land change or removed, the $R^2 = 0$ for $Q_{\text{river}}^{\text{d}} - Q_{\text{river}}^{\text{p}}$ and $R^2 = 0$ for $Q_{\text{river}}^{\text{d}}/Q_{\text{river}}^{\text{p}}$ and $R^2 = 0.09$ (P = 0.2) for $Q_{\text{river}}^{\text{d}}/Q_{\text{river}}^{\text{p}}$ using binned data, which is not statistically significant (Fig. 2a and Extended Data Fig. 1a). These results illustrate the need for caution with particularly influential outliers such as this one. Our concern is not only that the rates of change for individual deltas are inaccurate, but also that this inaccuracy permeates into the strong but unsupported conclusion regarding a global-scale link to human-induced sediment flux. This single misidentification led to the incorrect attribution of 16% of land change to human modifications. An $R^2 = 0$ excludes the possibility of a quantifiable link.

There are errors in the automatically generated buffers of Nienhuis et al.¹, as shown by our analysis of: (1) a random sample of 108 (1%) river mouths and (2) the 100 largest deltas in their dataset (Extended Data Fig. 2b,c). The correlation is very low to zero ($R^2$ of 0–0.13) with our operator-verified buffers. The incorrect land-change values are not limited to the Yellow River Delta. The automatically generated buffers of Nienhuis et al.¹ are often misplaced (therefore, a plethora of features was identified as deltas and deltas with misidentified land changes were included) and/or yield changes that are neither coastal nor driven by the processes proposed by Nienhuis et al.¹ (Extended Data Fig. 2 and Extended Data Table 1). The buffer that Nienhuis et al.¹ use to delimit the Yangtze, a major world delta, is misplaced by 100 km, yielding land
loss instead of a strong land gain. Replacing their land-change values with those identified by our buffers for the 100 largest deltas in the original (v.1) dataset yields bins that are the largest for decreased sediment flux ($Q_{river}^d/Q_{river}^p < 1$), revealing the effect of misidentified negative land change in large deltas, as shown by the magenta bins in Extended Data Fig. 1aii. The land-cover dataset¹² used by Nienhuis et al.¹ to mask anthropogenic delta transformations is inadequate, leading Nienhuis et al.¹ to overestimate—in populous Asian deltas—land loss (for example, the Mekong, Yangtze and Red River deltas) or land gain, corresponding to human artefacts (Pearl and Krishna deltas) (Extended Data Fig. 2d–k), such as the conversion of land to fishponds for aquaculture in the Yellow River Delta (Fig. 2b), which they recorded as natural land loss. Nienhuis et al.¹ carried out a check on the change in land area using 40 deltas in Madagascar and conclude their analysis has a standard error of 1%. Madagascar deltas are a biased choice and do not provide a random selection that is representative of a global-scale dataset. Massive deforestation of this island (a loss of 44% between 1953 and 2014)¹³ generates high sediment discharge that is favourable to delta land gain.

There are further inconsistencies in the successive datasets of Nienhuis et al.¹. In response to our concerns, Nienhuis et al.¹ generated a new dataset (v.2, March 2021) for the 100 largest deltas (Max100). This time, Nienhuis et al.¹ used a more-cautious, manual—rather than automatic—determination of land change. Adding the land change from the Max100 dataset results in a fivefold increase in land area (from 12 to 60 km² yr⁻¹), but there is no statistically significant relationship with the ratio of $Q_{river}^d/Q_{river}^p$ with the difference of $Q_{river}^d - Q_{river}^p$ or with the v.1 dataset (Extended Data Fig. 1b,c). This shows that an improved definition of the buffers is not sufficient to validate the finding of a net delta land gain that is attributable to humans.

Nienhuis et al.¹ use automatic buffers that do not accurately represent global deltas and their changes. Quantifying global delta land changes—especially in large populous deltas—requires clarity in their identification and analysis. Deltas are complex features, and the analysis of global-scale links of delta change with human actions requires the acquisition of accurate digital datasets and rigorous modelling. This Comment is relevant to analyses of coastal systems and landscape geomorphology that are faced with increasingly large global datasets and is, hopefully, of interest to a wide audience.

## Data availability

We used the original data provided by the corresponding author of Nienhuis et al.¹ and data¹ uploaded on GitHub (https://github.com/jhnienhuis/GlobalDeltaChange).

## Code availability

All of the MATLAB codes used for the data analysis and figure production have been uploaded to GitHub (https://github.com/FlorinZai/Global_Delta_Check). Interactive Google Earth Engine scripts are also provided as links in the GitHub repository. These can be used to compare our operator-derived buffers with the automatic buffers of Nienhuis et al.¹.

1. Nienhuis, J. H. et al. Global-scale human impact on delta morphology has led to net land area gain. *Nature* **577**, 514–518 (2020).

2. Nienhuis, J. H. et al. Author Correction: Global-scale human impact on delta morphology has led to net land area gain. *Nature* **608**, E13 (2022).

3. Syvitski, J. et al. Large deltas, small deltas: toward a more rigorous understanding of coastal marine deltas. *Glob. Planet. Change* **218**, 103958 (2022).

4. Besset, M., Anthony, E. J. & Bouchette, F. Multi-decadal variations in delta shorelines and their relationship to river sediment supply: an assessment and review. *Earth Sci. Rev.* **193**, 199–219 (2019).

5. Wright, L. D. in *Coastal Sedimentary Environments* 2nd edn (ed. Davis Jr, R. A.) 1–76 (Springer, 1985).

6. Evans, G. Deltas: the fertile dustbins of the world. *Proc. Geol. Assoc.* **123**, 397–418 (2012).

7. Donchyts, G. et al. Earth's surface water change over the past 30 years. *Nat. Clim. Change* **6**, 810–813 (2016).

8. Syvitski, J. P. M. et al. Sinking deltas due to human activities. *Nat. Geosci.* **2**, 681–686 (2009).

9. Giosan, L., Syvitski, J., Constantinescu, S. & Day, J. Climate change: protect the world's deltas. *Nature* **516**, 31–33 (2014).

10. Brondizio, E. et al. Catalyzing action towards the sustainability of deltas. *Curr. Opin. Environ. Sustain.* **19**, 182–194 (2016).

11. Li, C., Zhu, L., Dai, Z. & Wu, Z. Study on spatiotemporal evolution of the Yellow River delta coastline from 1976 to 2020. *Remote Sens.* **13**, 4789 (2021).

12. Bontemps, S. et al. GLOBCOVER 2009—Products Description and Validation Report. (ESA & UCL, 2011).

13. Vieilledent, G. et al. Combining global tree cover loss data with historical national forest cover maps to look at six decades of deforestation and forest fragmentation in Madagascar. *Biol. Conserv.* **222**, 189–197 (2018).

**Author contributions** All authors developed and contributed to drafts of the text and figures. The concept for this article was designed by E.A. and F.Z.; F.Z., F.T. and M.B. contributed to data analysis and acquisition. All authors contributed to the elaboration of the concepts presented here.

**Competing interests** The authors declare no competing interests.

**Additional information**

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s41586-022-05624-x.

**Correspondence and requests for materials** should be addressed to Florin Zăinescu.

**Reprints and permissions information** is available at http://www.nature.com/reprints.

**Publisher's note** Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© The Author(s), under exclusive licence to Springer Nature Limited 2023
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**Extended Data Fig. 1** | Statistical relationships further invalidating claim of delta land area gain due to humans.
Corrected Fig. 3a of Nienhuis et al.¹ (ai) is compared with the same sediment flux change ratio with different data corrections for the V1 dataset. R² of 0 with p = 0.0004 is obtained with x-axis values between 0 and 2 (92%) of the data. When the whole dataset is considered, p becomes non-significant (p = 0.98). A correction for the Yellow delta land changes results in R² of 0 or very close to 0, and a p-value that is not statistically significant for bins (aii, blue). Max100 land change (our data) replaced into V1 dataset shows R² = 0 and p that is not significant. Continuous lines show regressions for all the 'delta' points; dashed lines show regressions for the 20

bin averages; axes are extended to show the full variability of bins. Most of the large deltas fall beyond the chart's y axis. Comparison of the ratio $Q_{river}^d/Q_{river}^p$ with version 2 (V2) land changes (bi), and of the difference $Q_{river}^d - Q_{river}^p$ with V2 land changes of Nienhuis et al.¹ (bii); comparison of version 1 (V1, original) and V2 datasets of Nienhuis et al.¹ for Max100 deltas shows low agreement for all deltas (ci), and no agreement for 75 deltas within ±1 km² land change (cii); comparison of V2 dataset of Nienhuis et al.¹ with Max100 land change obtained through our data check and our buffers shows good agreement for all deltas (di), and fair agreement for 75 deltas within ±1 km² land change (dii).
108 random river mouths

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Max100 river mouths

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**Extended Data Fig. 2 | Exemplification of misidentified land changes.**
Distribution of our 108 random samples and the 100 largest deltas (Max100) on the sediment discharge continuum of Nienhuis et al.¹ (ai), and their global distribution (aii). Note biased error check of Nienhuis et al.¹ limited to Madagascar Island (aii) (see further comment on the Madagascar choice in the Supplementary Discussion); comparisons of land changes of Nienhuis et al.¹ with our 108 random samples (44 polygons, bi, bii- zoom on data), and with the Max100 deltas (ci, cii- zoom on data). Plots bii, cii show subsets of their

respective datasets, all with R² = 0. Examples of wrongly identified land changes by Nienhuis et al.¹ obtained on a selection of large deltas as a result of misplaced buffers (d, j), human-induced land transformations wrongly reported as natural land changes (e, f, h), and misidentified deltas (k). Original buffers (light grey) of Nienhuis et al.¹ are compared with our re-drawn data-check buffers (yellow) based on which only natural, and not human-transformed, delta coastal change should be calculated.
<table><caption>Extended Data Table 1 | Analysis of 108 randomly selected ‘river mouths’ from data of Nienhuis et al.¹ showing numerous discrepancies</caption><thead><tr><th>EE Delta</th><th>BasinID</th><th>Polygon</th><th>Delta</th><th>River mouth type</th><th>Anthrop.</th><th>Anthrop.</th><th>Interior</th><th>Small</th><th>Buffer</th><th>Obs.</th></tr><tr><td></td><td></td><td></td><td></td><td></td><td>filtering</td><td>change</td><td>buffer</td><td>overlap</td><td></td><td></td></tr></thead><tbody><tr><td>0</td><td>69</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>1</td><td>87861</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>1</td><td></td><td>River and estuarine dynamics</td></tr><tr><td>2</td><td>87825</td><td>0</td><td>0</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Tidal inlet</td></tr><tr><td>3</td><td>227807</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Dikes</td></tr><tr><td>4</td><td>299177</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>0</td><td></td><td></td></tr><tr><td>5</td><td>201340</td><td>0</td><td>0</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Port development, dikes</td></tr><tr><td>6</td><td>135103</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>0</td><td></td><td>Dike</td></tr><tr><td>7</td><td>103071</td><td>0</td><td>0</td><td>river mouth</td><td>NaN</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td>No mouth</td></tr><tr><td>8</td><td>75470</td><td>0</td><td>0</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>Rocky estuary, river mouth is far</td></tr><tr><td>9</td><td>250920</td><td>0</td><td>0</td><td>estuary(ría)</td><td>NaN</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td></td></tr><tr><td>10</td><td>135935</td><td>0</td><td>0</td><td>tidal inlets</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>11</td><td>170497</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>12</td><td>157003</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>No change</td></tr><tr><td>13</td><td>134898</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>1</td><td>1</td><td>Small buffer</td></tr><tr><td>14</td><td>162388</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Urban development</td></tr><tr><td>15</td><td>203988</td><td>0</td><td>0</td><td>anthropogenic river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>City of Nice</td></tr><tr><td>16</td><td>2769</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>17</td><td>574716</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>0</td><td></td><td></td></tr><tr><td>18</td><td>383601</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Urban development, dikes</td></tr><tr><td>19</td><td>7818</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>0</td><td></td><td>Dikes, ports, ships, shrimp farms</td></tr><tr><td>20</td><td>276923</td><td>0</td><td>0</td><td>estuary</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td></td><td>No river mouth</td></tr><tr><td>21</td><td>225</td><td>0</td><td>0</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Change from vegetation</td></tr><tr><td>22</td><td>151202</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>23</td><td>141533</td><td>1</td><td>1</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>24</td><td>3015</td><td>0</td><td>0</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>25</td><td>673450</td><td>0</td><td>0</td><td>estuary</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td></td><td></td></tr><tr><td>26</td><td>147976</td><td>0</td><td>0</td><td>estuary</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td></td><td></td></tr><tr><td>27</td><td>632150</td><td>1</td><td>1</td><td>delta</td><td>1</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>28</td><td>237383</td><td>0</td><td>0</td><td>estuary(tributary)</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td></td><td>Aquaculture, Shrimp farms</td></tr><tr><td>29</td><td>119979</td><td>0</td><td>0</td><td>estuary(tributary)</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td></td><td>No coast</td></tr><tr><td>30</td><td>145404</td><td>0</td><td>0</td><td>estuary</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>No coast</td></tr><tr><td>31</td><td>27687</td><td>0</td><td>0</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>32</td><td>472086</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>33</td><td>382322</td><td>0</td><td>1</td><td>delta</td><td>1</td><td>0</td><td>0</td><td>0</td><td></td><td>Urban development</td></tr><tr><td>34</td><td>124733</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Dike, change from vegetation</td></tr><tr><td>35</td><td>324962</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Salt ponds, shrimp farms</td></tr><tr><td>36</td><td>91673</td><td>0</td><td>1</td><td>river mouth</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td></td><td>Urban development</td></tr><tr><td>37</td><td>394758</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>1</td><td></td><td>No change</td></tr><tr><td>38</td><td>51818</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>No river mouth</td></tr><tr><td>39</td><td>689340</td><td>0</td><td>0</td><td>no river mouth</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td></td><td></td></tr><tr><td>40</td><td>295697</td><td>0</td><td>0</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>41</td><td>53848</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>42</td><td>422862</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>43</td><td>67218</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>44</td><td>378891</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Vegetation change</td></tr><tr><td>45</td><td>93</td><td>0</td><td>0</td><td>no river mouth</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td></td><td>No delta, Slope too steep, rocky</td></tr><tr><td>46</td><td>20123</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>No river mouth</td></tr><tr><td>47</td><td>286863</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>No delta, Slope too steep, rocky</td></tr><tr><td>48</td><td>264925</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>1</td><td></td><td>Shrimp ponds, dikes</td></tr><tr><td>49</td><td>249328</td><td>1</td><td>1</td><td>delta</td><td>1</td><td>0</td><td>1</td><td>0</td><td></td><td></td></tr><tr><td>50</td><td>146868</td><td>0</td><td>0</td><td>anthropogenic river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>100% anthropogenic</td></tr><tr><td>51</td><td>193701</td><td>0</td><td>0</td><td>no river mouth</td><td>NaN</td><td>NaN</td><td>NaN</td><td>0</td><td></td><td>Few pixels on bar estuary</td></tr><tr><td>52</td><td>132009</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>53</td><td>158128</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>54</td><td>211649</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>55</td><td>199306</td><td>0</td><td>0</td><td>anthropogenic river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Development</td></tr><tr><td>56</td><td>316765</td><td>0</td><td>0</td><td>no river mouth</td><td>0</td><td>0</td><td>0</td><td>1</td><td></td><td>Anthropogenic ponds</td></tr><tr><td>57</td><td>262538</td><td>1</td><td>1</td><td>delta</td><td>1</td><td>0</td><td>0</td><td>1</td><td></td><td></td></tr><tr><td>58</td><td>718850</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Didn't get split tip accumulation</td></tr><tr><td>59</td><td>94741</td><td>0</td><td>1</td><td>no river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>60</td><td>412843</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>1</td><td></td><td></td></tr><tr><td>61</td><td>638519</td><td>0</td><td>0</td><td>river mouth</td><td>NaN</td><td>NaN</td><td>NaN</td><td>1</td><td></td><td>Fjord?</td></tr><tr><td>62</td><td>216515</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Small buffer</td></tr><tr><td>63</td><td>147544</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Doesn't look like a delta</td></tr><tr><td>64</td><td>420134</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>River dynamics, coastal works</td></tr><tr><td>65</td><td>674974</td><td>0</td><td>1</td><td>delta</td><td>1</td><td>0</td><td>1</td><td>0</td><td></td><td></td></tr><tr><td>66</td><td>139009</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>67</td><td>213083</td><td>0</td><td>0</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Urban development, same delta as #97</td></tr><tr><td>68</td><td>126813</td><td>0</td><td>1</td><td>no river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>No river mouth</td></tr><tr><td>69</td><td>26952</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>70</td><td>145789</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>71</td><td>160787</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>72</td><td>25943</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>73</td><td>145</td><td>0</td><td>0</td><td>no river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>74</td><td>32721</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Srtn high coastal elevation</td></tr><tr><td>75</td><td>136526</td><td>0</td><td>1</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>No river mouth</td></tr><tr><td>76</td><td>40547</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Barrier diverted river mouth</td></tr><tr><td>77</td><td>71749</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>1</td><td></td><td>Srtn high coastal elevation</td></tr><tr><td>78</td><td>292724</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>0</td><td></td><td>Small &Misplaced buffer</td></tr><tr><td>79</td><td>48430</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>1</td><td></td><td></td></tr><tr><td>80</td><td>42223</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>81</td><td>259998</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>82</td><td>63338</td><td>0</td><td>0</td><td>tidal inlet</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>83</td><td>262402</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>1</td><td></td><td></td></tr><tr><td>84</td><td>275141</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>85</td><td>34059</td><td>0</td><td>0</td><td>no river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>River dynamics,Inside bigger delta</td></tr><tr><td>86</td><td>30270</td><td>0</td><td>0</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>87</td><td>395256</td><td>0</td><td>1</td><td>nondelta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>seasonal river mouth</td></tr><tr><td>88</td><td>654755</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td></td></tr><tr><td>89</td><td>634660</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>1</td><td></td><td></td></tr><tr><td>90</td><td>165985</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>0</td><td></td><td>Buffer overlap</td></tr><tr><td>91</td><td>323297</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Barrier diverted river mouth</td></tr><tr><td>92</td><td>373957</td><td>0</td><td>0</td><td>nondelta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Jetties at mouth</td></tr><tr><td>93</td><td>36503</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Tidal inlet</td></tr><tr><td>94</td><td>149</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>0</td><td></td><td>Estuary</td></tr><tr><td>95</td><td>331379</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>1</td><td>0</td><td></td><td>Small, misplaced buffer</td></tr><tr><td>96</td><td>39958</td><td>1</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>River dynamics</td></tr><tr><td>97</td><td>213107</td><td>0</td><td>1</td><td>delta</td><td>1</td><td>0</td><td>0</td><td>0</td><td></td><td>River dynamics, vegetation change</td></tr><tr><td>98</td><td>598116</td><td>0</td><td>1</td><td>nondelta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Urban development</td></tr><tr><td>99</td><td>630689</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>1</td><td></td><td>No river mouth, tidal creek</td></tr><tr><td>100</td><td>620926</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Urban development</td></tr><tr><td>101</td><td>416223</td><td>0</td><td>1</td><td>delta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>River dynamics</td></tr><tr><td>102</td><td>75078</td><td>0</td><td>0</td><td>canals</td><td>0</td><td>0</td><td>0</td><td>1</td><td></td><td>No river mouth, delta restoration</td></tr><tr><td>103</td><td>730746</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>1</td><td></td><td>Aquaculture, shrimp</td></tr><tr><td>104</td><td>583610</td><td>0</td><td>0</td><td>ocean creek</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>Barrier diverted river mouth</td></tr><tr><td>105</td><td>128186</td><td>0</td><td>0</td><td>nondelta</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>No delta, not linked to any real river</td></tr><tr><td>106</td><td>77603</td><td>1</td><td>1</td><td>delta</td><td>1</td><td>0</td><td>1</td><td>0</td><td></td><td>No river mouth.Dikes</td></tr><tr><td>107</td><td>599760</td><td>0</td><td>0</td><td>river mouth</td><td>0</td><td>0</td><td>0</td><td>1</td><td></td><td>Aquaculture, Inside bigger delta</td></tr><tr><td>Percentage(%)</td><td>41</td><td>53</td><td>14</td><td>0</td><td>19</td><td>8</td><td>25</td><td></td><td></td><td>Barrier diverted river mouth</td></tr></tbody></table>

Delta number is EE Delta. Presence is marked 1, absence marked 0; no data due to the absence of a delta is NaN.
# Reply to: Concerns about data linking delta land gain to human action

https://doi.org/10.1038/s41586-022-05625-w

Published online: 8 February 2023

<figure><img src="image_20.png" /></figure>

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J. H. Nienhuis<fnref n="1" />,², A. D. Ashton², D. A. Edmonds<fnref n="3" />, A. J. F. Hoitink⁴, A. J. Kettner<fnref n="5" />, J. C. Rowland⁶
& T. E. Törnqvist<fnref n="7" />

REPLYING TO F. Zăinescu et al. Nature https://doi.org/10.1038/s41586-022-05624-x (2023)

In the accompanying Comment, Zăinescu et al.<fnref n="1" /> raise a series of concerns about our study² that broadly relate, first, to our definition of what constitutes a delta and, second, to the link between human-impacted fluvial sediment flux and the change in the area of deltas. Here we show that our conclusions remain fully supported by the best-available data. Moreover, in our view, the concerns raised by Zăinescu et al.<fnref n="1" /> are unrelated to the primary scientific contribution of our paper: the first fully quantitative prediction of the continuum of delta shapes.

Regarding the first concern, Zăinescu et al.<fnref n="1" /> propose that our definition for deltas is too broad. What defines a river delta has long been debated in the literature (since at least the 1960s<fnref n="3" />⁻⁶); how to define a delta was certainly integral to our own discussions, as we developed our framework to predict delta morphology based on driving factors. Our study uses a broad definition of what we categorize as a delta: an alluvial coastal morphology containing sediment delivered by a river. In our original study², we directly acknowledge that such a definition represents “a broader definition compared to other studies”, because it is unconventional. We welcome this opportunity to further clarify our choices. It is important to note, however, that our conclusions are not dependent on our definition of what constitutes a delta.

We took a broad view of deltas for two reasons. First, previous studies<fnref n="7" />,⁸ showed that, physically, coastal landforms appear in a continuum from very small to very large without a clear distinction that would separate large, well-studied deltas from strandplains or alluvial estuaries. In other words, there is, so far, no objective threshold that determines when enough sediment has accumulated at a river mouth for it to become a delta. A previous study⁶ considered a landform deltaic if more than 80% of its sedimentary mass is river-derived, but that study did not define a minimum mass. Within our framework of predicting delta morphology from external driving factors, we analysed alluvial river mouths for which, at least, an arbitrary quantity of sediment (0.01 kg s⁻<fnref n="1" />) is expected to be delivered to the river mouth; certainly, for these mouths, river-derived sediments would be expected to be deposited nearshore, including subaqueous deposits with no subaerial expression. Such deposition is, arguably, the minimum requirement for a delta⁴,⁹. As Zăinescu et al.<fnref n="1" /> note, two of us have worked on this problem before¹⁰; in that study, we used a different arbitrary (and more restrictive) threshold for a subaerial expression (for example, delta protrusion must exceed five times the channel width) to define when a river mouth becomes a delta.

Second, our approach differs from the previous study¹⁰ because we studied changes in sediment delivery to deltas in Nienhuis et al.². Increases in river sediment flux can cause strandplains to grow into ‘traditional’ deltas (referred to herein as geomorphic deltas), whereas

decreases in sediment flux force fluvial networks to become more tide-influenced and potentially cause a transition to alluvial estuaries. Our broad definition allows us to capture landforms throughout these morphological transitions and thus improve predictions of delta change.

The resulting approach allows us to quantitatively predict, using two non-dimensional numbers, what type of morphology would be expected at more than 10,000 river mouths. Our methodology successfully predicts that strandplains or estuaries would be found at many river mouths, including those identified along the British coastline by Zăinescu et al.<fnref n="1" />. These river mouths could expand subaerially into a geomorphic delta with increases in sediment supply or owing to decreases in tidal amplitudes.

Notably, our main findings do not depend on our definition of what constitutes a delta, or on the accidental inclusion of any non-deltaic river mouths. As discussed below, when we restrict our analysis to only those landforms that meet the criteria of a geomorphic delta, as manually selected in a previous study¹⁰, we still see that human-induced changes to sediment flux have affected delta growth (Fig. 1b).

Regarding the second concern, Zăinescu et al.<fnref n="1" /> suggest that our analysis of land area changes does not show that human-induced changes to sediment flux have led to delta growth. They demonstrate that a large part of the Pearson's R² was explained by one data point, the Yellow River Delta, for which some of the land-to-water change was because of the development of aquaculture and not because of delta morphodynamics. We thank Zăinescu et al.<fnref n="1" /> for pointing this out. Taking out the Yellow River Delta indeed reduces the R² substantially. The Pearson's R² correlation is a statistic that assesses the fit of a linear model and, as described previously¹¹, such models remain sensitive to individual data points that may sometimes be inaccurate. We agree that a simple linear model is unlikely to be the best approach to statistically quantify a large, noisy dataset such as the one we present. However, our finding that global land changes in deltas are related to human activities is confirmed by multiple lines of evidence and is not affected by our correction¹².

The simplest basis for our conclusion that human-induced changes to sediment flux have altered delta growth (as presented in Table 1 of our previous study²) is that deltas with declining fluvial sediment supplies show altered changes in land area compared with deltas that have increasing sediment supplies and that are, on average, growing. As we reported in our previous study², 25% of the global net land gains (around 14 km² yr⁻<fnref n="1" />) can be attributed to increasing sediment supply from deforestation. We concluded that human impacts had led to net land area gains.

Further to the evidence that we reported previously², other statistical tests lead to the same conclusion. A Spearman rank correlation coefficient, which is insensitive to outliers, shows that there is a

<footnote n="1">¹Department of Physical Geography, Utrecht University, Utrecht, The Netherlands. ²Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.</footnote>

<footnote n="3">³Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, IN, USA. ⁴Environmental Sciences, Wageningen University & Research, Wageningen, The Netherlands.</footnote>

<footnote n="5">⁵Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, USA. ⁶Earth & Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM, USA.</footnote>

<footnote n="7">⁷Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA, USA. ⁸e-mail: j.h.nienhuis@uu.nl</footnote>
<figure><img src="image_21.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>e</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>f</figcaption></figure>

<figure><img src="image_26.png" /></figure>

Human-induced sediment flux change ($Q_{river}^{d}/Q_{river}^{p}$)

**Fig. 1 | Change in delta area for decreasing, stable and increasing fluvial sediment fluxes.** a, All of the data as presented in ref. ². b, A selection of the data using the delta definition of a previously published study¹⁰. c, All of the data except the Yellow River Delta. d–f, Analyses with smaller (d) or larger (e) buffers to delineate the deltas or with different anthropogenic land-cover

masks¹²(f). P values are calculated using Kolmogorov–Smirnov (KS) tests to quantify the difference between the decreasing (less than –50%) and increasing (more than 50%) sediment fluxes and using Spearman rank correlations of the raw (unbinned) data. Stable sediment fluxes are indicated as ‘–50% to 50%’. For further details, see Supplementary Information. Data are mean ± s.e.m.

statistical dependence between fluvial sediment flux changes and delta land area change, with gains exceeding losses (Fig. 1). Other general statistical tests for comparing categorical, binned data—such as the Kolmogorov–Smirnov test—also suggest that deltas with increasing sediment supplies have elevated growth rates compared with deltas with decreasing sediment supplies. These results are robust and do not depend on the inclusion or exclusion of the Yellow River Delta (Fig. 1c), or whether we investigate the difference ($Q_{river}^{d} - Q_{river}^{p}$) or the fractional change ($Q_{river}^{d}/Q_{river}^{p}$) in the disturbed compared with the pristine sediment supplies (Supplementary Table 1).

We next investigated whether the presence of many small or several large deltas affected our analysis. Reanalysis using only the (larger) deltas from a previous study¹⁰ confirms our previous conclusions (Fig. 1b). Removing the upper and lower 5% percentile of the land-area change data (the Yellow River Delta and most other large deltas, accounting for 76% of the global fluvial sediment flux; Supplementary Fig. 3) also does not change the significance of the test. Thus, our additional analyses confirm that human-induced changes to the fluvial sediment flux have led to gains in delta land area.

Our results are based on automated methods of delta delineation and buffer selection, which we prefer to bespoke, non-automated methods. Non-automated methods, such as the ones used by Zăinescu et al.¹, might work better in specific cases, but we think that they rely on many arbitrary decisions that vary between deltas and are difficult to reproduce objectively. The resulting conclusion in our paper that land area changes are affected by sediment flux changes are robust and do not depend on the exact size or type of delta buffers (Fig. 1d,e). Furthermore, when improving our original analysis by using a new

land-cover classification¹³ that more accurately removes anthropogenic land conversions, we still see that human-induced changes to the fluvial sediment flux have altered delta growth (Fig. 1f and Supplementary Fig. 2).

As observations and modelling of the Earth's surface become more sophisticated over the coming decades, the land change analysis that we published previously² will continue to be reworked—the subject of future studies the findings of which we look forward to. Although our additional analyses again demonstrate that human-driven sediment flux change has led to gains in delta land area (Fig. 1), we assert that the prediction of delta morphology (a quantification of a previously published qualitative diagram¹⁴) represents the key scientific advance of our paper. Not only is this keystone aspect of our work not refuted by Zăinescu et al.¹, it is barely mentioned. For example, our analysis predicts that geomorphic deltas would not be expected in their example of the well-studied British Isles, and we quantify our expectation. As much as we anticipate revised land change analyses, we avidly endorse refinement of our quantitative prediction of delta form.

## Data availability

Data to reproduce Fig. 1 and Supplementary Figs. 1–3 are available on https://github.com/jhnienhuis/GlobalDeltaChange under pub_figures.

## Code availability

Code to reproduce Fig. 1 and Supplementary Figs. 1–3 is available on https://github.com/jhnienhuis/GlobalDeltaChange under pub_figures.
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**Acknowledgements** J.H.N. was supported by US National Science Foundation award EAR1810855 and the Netherlands Organisation for Scientific Research (NWO) (vi.veni.192.123).

**Author contributions** J.H.N. produced the figures. J.H.N., A.D.A., D.A.E., A.J.F.H., A.J.K., J.C.R. and T.E.T. discussed the results and contributed to the text.

**Competing interests** The authors declare no competing interests.

**Additional information**

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s41586-022-05625-w.

**Correspondence and requests for materials** should be addressed to J. H. Nienhuis.

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