# Global trends in water and sediment fluxes of the world's large rivers

Li Li, Jinren Ni, Fang Chang, Yao Yue, Natalia Frolova, Dmitry Magritsky, Alistair G.L. Borthwick, Philippe Ciais, Yichu Wang, Chunmiao Zheng, et al.

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Li Li, Jinren Ni, Fang Chang, Yao Yue, Natalia Frolova, et al. Global trends in water and sediment fluxes of the world's large rivers. Science Bulletin, 2020, 65 (1), pp.62-69. 10.1016/j.scib.2019.09.012. hal-04233214

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**Citation for published version:**

Li, L, Ni, J, Chang, F, Yue, Y, Frolova, N, Magritsky, D, Borthwick, A, Ciais, P, Wang, Y, Zheng, C & Walling, D 2020, 'Global trends in water and sediment fluxes of the world's large rivers', *Science Bulletin*, vol. 65, no. 1, pp. 62-69. https://doi.org/10.1016/j.scib.2019.09.012

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10.1016/j.scib.2019.09.012

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Article

Global trends in water and sediment fluxes of the world's large rivers

Li Li<sup>a</sup>, Jinren Ni<sup>a,*</sup>, Fang Chang<sup>a</sup>, Yao Yue<sup>b</sup>, Natalia Frolova<sup>c</sup>, Dmitry Magritsky<sup>c</sup>, Alistair G.L. Borthwick<sup>d</sup>, Philippe Ciais<sup>e</sup>, Yichu Wang<sup>a,f</sup>, Chunmiao Zheng<sup>g</sup>, Desmond E. Walling<sup>h</sup>

ARTICLE INFO

Article history:

Received 1 August 2019

Received in revised form 16 August 2019

Accepted 19 August 2019

Available online xxxx

Keywords:

Water and sediment

Global trend

Co-varying pattern

Cause

Large river

A B S T R A C T

Water and sediment transport from rivers to oceans is of primary importance in global geochemical cycle. Against the background of global change, this study examines the changes in water and sediment fluxes and their drivers for 4307 large rivers worldwide (basin area ≥1000 km²) based on the longest available records. Here we find that 24% of the world's large rivers experienced significant changes in water flux and 40% in sediment flux, most notably declining trends in water and sediment fluxes in Asia's large rivers and an increasing trend in suspended sediment concentrations in the Amazon River. In particular, nine binary patterns of changes in water-sediment fluxes are interpreted in terms of climate change and human impacts. The change of precipitation is found significantly correlated to the change of water flux in 71% of the world's large rivers, while dam operation and irrigation rather control the change of sediment flux in intensively managed catchments. Globally, the annual water flux from rivers to sea of the recent years remained stable compared with the long-time average annual value, while the sediment flux has decreased by 20.8%.

© 2019 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.

1. Introduction

Changing fluvial water and sediment fluxes exert a key effect on river ecosystems [1,2] and their management [3]. Such changes represent critical aspects of global change [4], also affecting delta evolution and sustainability [5,6] and aquatic organisms' habitats [7–9]. Much literature currently exists on the separate trends in water or sediment flux associated with individual rivers [5] or groups of rivers [10,11] due to the limited availability of long-term observational data [12] for coupled records on both water and sediment, particularly for developing countries [13–15]. Walling and Fang [13] analyzed trends of water and sediment fluxes before the 1990s, but their analysis did not extend to rivers in South America, Africa and Oceania. To reveal the rare investigated co-varying trends in addition to the recent trends of water and sediment fluxes in world's large rivers, we compiled long-term records of annual water flux (discharge Q) from 8089 gauging stations

located in 4307 rivers (all with a basin area ≥1000 km²), with roughly one fourth of the gauge stations being at the mouth of rivers (Supplementary Data S1 online). The longest record dates from 1806 for the Dresden Station on the Elbe River, and about half of the stations provided records of annual water flux spanning 40 years or more. Information on annual mean suspended sediment concentration (C<sub>SS</sub>) and annual suspended sediment flux (Q<sub>SS</sub>) was also collected for 309 of the rivers (516 stations), including stations at the river mouth for 129 rivers (496 stations; Supplementary Data S1 online). The data cover 88% of the land surface, and 60% of the area actively contributing water flux to the oceans [16,17].

Non-parametric trend analysis based on the longest recorded data series (see Method) was used to identify significant trends in water and sediment fluxes for individual rivers. Furthermore, the co-varying trends under the impacts of climate change and human activities were presented under a Sudoku framework. It should be noted that the trend analysis requires accounting for the differences in the length of available records to guarantee the reliability and nature of the results. Long duration records can

<footnote><sup>a</sup>Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China</footnote>

<footnote><sup>b</sup>State Key Laboratory of Water Resources and Hydropower Engineering Science, School of Water Resources and Hydropower Engineering, Wuhan University, Wuhan 430072, China</footnote>

<footnote><sup>c</sup>Department of Hydrology, Faculty of Geography, Moscow State University, Moscow 119991, Russia</footnote>

<footnote><sup>d</sup>Institute of Infrastructure and Environment, School of Engineering, The University of Edinburgh, The King's Buildings, Edinburgh EH9 3JL, UK</footnote>

<footnote><sup>e</sup>Laboratoire des Sciences du Climat et de l'Environnement, IPSL, CEA, CNRS, UVSQ, Gif-sur-Yvette 91191, France</footnote>

<footnote><sup>f</sup>Beijing Innovation Center-Engineering Science & Advanced Technology, Peking University, Beijing 100871, China</footnote>

<footnote><sup>g</sup>School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China</footnote>

<footnote><sup>h</sup>Department of Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UK</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: jinrenni@pku.edu.cn (J. Ni).</footnote>
provide more definitive evidence of long-term trends, whereas records covering shorter, more recent periods may provide a clearer indication of contemporary or recent changes. Here, the longest available record for each river was used for trend analysis. Results that compare the magnitude of water flux (Q) and sediment flux (Q<sub>SS</sub>) in the most recent decade with the longer-term mean were also presented. Based on the reliability analysis on record length in terms of guarantee rate, we suggest that a minimum of 30 years (either continuous or not) is required to reflect the changing trend in a longer period (the recent 40 years, 1971–2010).

## 2. Materials and methods

LOADEST [18,19] was used to evaluate the sediment flux at 123 stations (25% of the 496 stations on the 309 large rivers) using continuously observed daily water flux data and discretely measured sediment flux data on daily based. For each station, the water discharge and suspended sediment concentration time series associated with a given year were used to calibrate regression models that permit the values of daily sediment flux using formulas fitted to the water discharge time series [18]. The Adjusted Maximum Likelihood Estimation (AMLE) method was applied during calibration, and the regression model with lowest Akaike Information Criterion (AIC) value was selected to estimate sediment flux. R² values associated with the models fitted to the data from individual stations on the annual basis were presented in Supplementary Data S2 (online).

Trends in Q, C<sub>SS</sub>, and Q<sub>SS</sub> were analyzed by means of the non-parametric Mann-Kendall (MK) test [20,21]. Trend-free pre-whitening (TFPW) was used to ensure data independence before using the MK test [22].

Pre-human sediment fluxes were estimated for the rivers included in the sediment flux database using the Area Relief Temperature sediment delivery model (ART) [23].

Data Sources and detail of Methods were given in Supplementary materials (online).

## 3. Results and discussions

### 3.1. Global trends in water and sediment variations of large rivers

The records of annual water flux (Q) show no significant change for 3301 (77%) rivers, decreased significantly for 500 rivers (11%) and increased for 506 rivers (12%) rivers (Fig. 1a). The proportions of rivers experiencing significant changes in Q are the highest in Africa and North America (Table S1 online). In Fig. 1a, global distribution map of Q trends is shown with all gauging stations. Stations with decreased Q are mainly located in the central and western Africa, eastern Asia, southern Europe, western North America and eastern Australia. Stations with increased water flux are mainly located in northern Asia, northern Europe, and northern and eastern North America.

Analysis of all the C<sub>SS</sub> records indicate significant reductions in suspended sediment concentrations for 105 rivers (34%) and an increase in only 22 rivers (7%). The majority of the remaining rivers show no significant trend (Fig. 1b). Most of rivers with significant changes in C<sub>SS</sub> are in developing regions (Africa, Asia and South America) (Table S1 online). Rivers with significant decreases in C<sub>SS</sub> are in eastern Asia and North America, and those with significant increases are in northeastern China, eastern North America and South America.

The majority (82%) of rivers exhibit the trends of the same sign for Q<sub>SS</sub> and C<sub>SS</sub>. The sediment flux Q<sub>SS</sub> = C<sub>SS</sub> × Q shows no significant change for 196 (63%) of rivers, with the other 100 (32%) recorded decreases in Q<sub>SS</sub>. An increased sediment flux is only observed for

13 (4%) of rivers (Fig. 1c). 72% of rivers in Asia show significant upward or downward trends in Q<sub>SS</sub>, and less than 20% (Table S1 online) in North America and Oceania. Stations with decreasing trends in Q<sub>SS</sub> are widely distributed in North America and Eastern Asia. Increased Q<sub>SS</sub> is mostly evident in the Northeastern China and the north of South America.

### 3.2. Basic patterns for water-sediment co-varying trends and their causes

Fig. 2a divides the trends in [Q, Q<sub>SS</sub>] combination across all rivers into a matrix of nine basic patterns, each according to whether the variable is in stable, decreasing, or increasing. The histogram shows that water and sediment fluxes measured at 138 (45%) rivers belong to Pattern I, with no significant trend in both fluxes. The Pattern II trend of stable Q and decreasing Q<sub>SS</sub> covers 50 (16%) rivers, and the Pattern V trend of both decreasing Q and Q<sub>SS</sub> covers 41 (13%) rivers. Compared to Walling and Fang's study [13] which represented past trends in [Q, Q<sub>SS</sub>] based on a smaller dataset (only in Europe, Asia, and North America), changes to [Q, Q<sub>SS</sub>] are evident in the present data, as well as a new trend pattern (decrease in water flux with increase in sediment flux).

In all continents, the dominant [Q, Q<sub>SS</sub>] pattern of change is Pattern I, except in Asia, which contains a mix of Pattern I, II and V. Rivers exhibiting Pattern II are very common worldwide, except for North America and Africa. Pattern III representing stable Q and increasing Q<sub>SS</sub> is mainly found in South America and Pattern IV representing decreasing Q and stable Q<sub>SS</sub> is found in North America and Oceania. Looking globally, Q<sub>SS</sub> has a general stable or decreasing trend in developed regions, whereas Patterns III and IX associated with an increasing trend in Q<sub>SS</sub> occur in developing regions, except Asia. Patterns V mainly occur in Asia and Africa, but Pattern VII representing increasing Q and stable Q<sub>SS</sub> only predominates in North America and Europe. Trends in water and sediment fluxes are exemplified by the representative large rivers for the nine basic [Q, Q<sub>SS</sub>] patterns in Fig. 2b, including the famous large rivers such as Amazon River, Rhine River, Mississippi River, Yangtze River, Yellow River, Yana River, and Blue Nile River.

To examine the impacts of Asian rivers undergoing the most intensive human interference in recent decades, we compared the statistical results with and without these rivers. It is found that the number of rivers associated with Pattern I does not change noticeably; instead, occurrences of Patterns V and II reduce by 68% and 48% respectively (Fig. 3a), indicating that the reduction in Q<sub>SS</sub> is widespread in rapidly developing Asian countries where numerous dams have been constructed in recent years. The long-term effect of human activities on Q<sub>SS</sub> has been estimated using the Area Relief Temperature sediment delivery model [23] (ART) which can provide a reliable estimate (overall R² of the 461 rivers equals 0.99, Table S2 online) of the pre-human sediment flux (i.e. natural sediment flux before there was dominant human disturbance) of a river basin. A comparison of these estimates with the observed long-term mean Q<sub>SS</sub> in most (71%) large rivers in Asia has decreased compared to pre-human Q<sub>SS</sub> values (below the 1:1 line in Fig. 3b).

Over the recent decade (2000–2010), the [Q, Q<sub>SS</sub>] trends for global rivers have changed substantially compared to the previous decades. The proportion of rivers showing Pattern I is decreasing, whereas the proportions showing with Patterns II and V are increasing (Fig. 3c). These changes primarily resulted from the great reduction of Q and Q<sub>SS</sub> (Fig. 3d) in Asian rivers after 2000 owing to dam constructions.

In seeking to establish the key factors responsible for producing the nine different patterns of [Q, Q<sub>SS</sub>] changes and their global distribution, both climate and human impacts have been identified as important controls. In arid regions, the water discharge data for the
<figure><img src="image_5.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 1.** Trends in water discharge Q for 8089 hydrologic stations located in 4307 of the world's large rivers, and suspended sediment concentration $C_{SS}$ and suspended sediment flux $Q_{SS}$ for 496 stations on the 309 large rivers color coded according to basin climate conditions and bounded by dashed lines (left plots); and global spatial distributions of trends in Q, $C_{SS}$ and $Q_{SS}$ (right panels). (a) Q. (b) $C_{SS}$. (c) $Q_{SS}$. All trends are significant at the 0.05 level.</figcaption></figure>

majority (81%) of stations demonstrate either a significant decreasing trend, or a decreasing but not statistically significant trend. In humid regions, most stations (71%) show a stable long-term trend. Compared with water flux, $C_{SS}$ seems less sensitive to background climate conditions, but primarily controlled by human activities. When Activity of type A (e.g. dam construction, irrigation and implementation of soil and water conservation programs) is implemented, $C_{SS}$ is significantly reduced. However, Activity of type B (e.g. deforestation, mining, and land clearance) coincides increased $C_{SS}$, which is particularly common in developing countries.

The trend in $Q_{SS}$ is determined by changes in Q and $C_{SS}$. Only 59% of stations have consistent trends in both $Q_{SS}$ and Q, whereas 82% have consistent trends in $Q_{SS}$ and $C_{SS}$. This suggests that $C_{SS}$ is the dominant factor explaining changes of $Q_{SS}$. However, changing Q also plays a role, and thus the trends in [Q, $Q_{SS}$] are more diverse than for [Q, $C_{SS}$]. In humid regions, stable $Q_{SS}$ could be sustained

provided that there is sufficient river discharge even if $C_{SS}$ fluctuates. In contrast, there is a decreasing trend in $Q_{SS}$ in arid areas due to reduced Q, even when $C_{SS}$ remains stable.

For the most commonly occurring [Q, $Q_{SS}$] patterns, Fig. 4 describes river responses to different anthropogenic impacts in various climatic regions (mainly across the non-glaciered surface). A natural river basin with stable annual rainfall will retain stable trends in [Q, $Q_{SS}$]. When human activities occur, the [Q, $Q_{SS}$] pattern of rivers in different climate zones will be altered in different ways. Activity A will make most rivers in humid regions exhibit Pattern II behavior, and Pattern V behavior in arid regions. In contrast, Activity B will cause the majority of rivers in humid and arid regions to be characterized by Pattern III and Pattern IV behavior, respectively.

The trend in water flux affected by human activities must be fully considered. Soil and water conservation measures intercept
<table><caption>(a)</caption><thead><tr><th>Q</th><th>Stable</th><th>Decrease</th><th>Increase</th></tr></thead><tbody><tr><th>Q<sub>ss</sub></th><td></td><td></td><td></td></tr><tr><th>Stable</th><td>Pattern I S-S</td><td>Pattern IV D-S</td><td>Pattern VII I-S</td></tr><tr><th>Decrease</th><td>Pattern II S-D</td><td>Pattern V D-D</td><td>Pattern VIII I-D</td></tr><tr><th>Increase</th><td>Pattern III S-I</td><td>Pattern VI D-I</td><td>Pattern IX I-I</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>**Fig. 2.** Nine basic [Q, Q<sub>ss</sub>] patterns. (a) Matrix of nine pattern representing the changes of [Q, Q<sub>ss</sub>]. (b) Representative large rivers for nine different [Q, Q<sub>ss</sub>] patterns showing water flux (solid line) and sediment flux (histogram) time histories and corresponding trends (dashed line): (1) Rhine River at Rhenish Massif, (2) Yangtze River at Datong, (3) Amazon River at Obidos, (4) Songhuajiang at Jiamusi, (5) Yellow River at Lijin, (6) Nenjiang at Jiangqiao, (7) Yana River at Ubileynaya, (8) Mississippi River at Tarbert Landing, and (9) Blue Nile River at El Diem. All trends are significant at the 0.05 level.</figcaption></figure>

precipitation, increase infiltration, and enhance evaporation, thereby decreasing Q [24]. Intensive water abstraction from rivers further reduces Q and thus modifies its trend [14]. Meanwhile, deforestation or the expansion of agriculture may lead to increasing Q [25].

Unlike ordinary rivers, an increased trend in river flux may occur in the relatively humid high-altitude Arctic river, such as the Yenisey River. This may be caused by increased runoff from

rainfall or retreating glaciers responding to global warming [26,27], but may also reflect measurement uncertainty [28].

### 3.3. Quantitative impacts of global climate change and human activities on trends in water and sediment fluxes

Quantitative evaluation has been made on the impacts of global climate change and typical human activities on changing trends in
<figure><img src="image_9.png" /><figcaption>**Fig. 3.** Asian rivers' contribution on [Q, Q<sub>SS</sub>] trends for world's large rivers over past decade and comparison of observed and estimated prehuman sediment fluxes. (a) River numbers according to [Q, Q<sub>SS</sub>] patterns, with and without Asian rivers. (b) Comparison between pre-human and observed Q<sub>SS</sub>; the Asian rivers' names are: 1, Amguema; 2, Kolyma; 3, Alazeya; 4, Yana; 5, Lena; 6, Anabar; 7, Yenisei; 8, Taz; 9, Ob; 10, Indus; 11, Ganges; 12, Chao-Phraya; 13, Mekong; 14, Zhujiang; 15, Yangtze; 16, Minjiang; 17, Qiantangjiang; 18, Huaihe; 19, Yellow; 20, Haihe; 21, Liaohe; 22, Xiaolinghe; 23, Yalujiang; 24, Sakarya. (c) Rivers' number percentage according to [Q, Q<sub>SS</sub>] patterns, with trends determined for two periods. (d) Comparison between pre-human and observed Q<sub>SS</sub>; the river names are: 1, Lena; 2, Yenisei; 3, Ob; 4, Severnaya-Dvina; 5, Danube; 6, Rhine; 7, Ebro; 8, Nile; 9, Kuban; 10, Indus; 11, Chao-Phraya; 12, Mekong; 13, Zhujiang; 14, Yangtze; 15, Minjiang; 16, Qiantangjiang; 17, Huaihe; 18, Yellow; 19, Xiaolinghe; 20, Liaohe; 21, Yalujiang; 22, Fitzroy; 23, Kenai; 24, San-Joaquin; 25, Mississippi; 26, Hudson; 27, Rio-Grande; 28, Amazon; 29, Parana; 30, Elqui; 31, Itata; 32, Biobio. Colored areas in inset-maps in b and d denote large river basins.</figcaption></figure>

water and sediment fluxes in the world's large rivers. Significant correlation between water flux and precipitation (P < 0.05) is found in 71% (3050) of world's large rivers, whereas only 33% (101) of large rivers show significant correlation between sediment flux and precipitation. In Fig. 5a, precipitation explains over 80% of the changes in water flux [4] of large rivers located in northern Asia and northern Europe where impact of human activities is relatively limited. However, precipitation explains less than 30% of the variations in water flux of rivers in the mid-west of North America, eastern Africa, southern Africa, eastern Asia, and southeastern Asia experiencing large-scale dam construction and/or over-irrigation. On the other hand, precipitation variation explains less than 30% to the changes in sediment flux in most rivers globally, except for north of Asia, Europe, and eastern North America (Fig. 5b). In contrast, less large rivers (23%) show significant correlations between temperature and water/sediment fluxes.

To distinguish human impacts from climate change, we used the Degree of Regulation Index (DOR, defined as the ratio of total storage capacity of all upstream dams to total discharge of the river, Fig. S1 online) related to dams, the Irrigation Index (defined as the ratio of irrigation area to virgin area contributing to the mean annual discharge, Fig. S2 online), precipitation, and

temperature. Of the 1931 rivers with at least 40-year records, we find that higher DOR and Irrigation Index correspond to lower correlation between annual water discharge and precipitation/temperature, mainly in river basins where DOR >20 and Irrigation Index >140 (Fig. 5c). This suggests that changing trends in water flux in these areas are altered primarily by intensive human disturbance. Similarly, lower correlation (P ≥ 0.05) between sediment flux and precipitation would be expected (Fig. 5d) upon DOR >8 and irrigation index >60 being met for concerned rivers. In general, sediment flux seems more sensitive to human interference than water flux in large rivers.

### 3.4. Water and sediment fluxes to the coastal ocean

We estimate that the total quantities of water flux Q and sediment flux Q<sub>SS</sub> are 31,629 km<sup>3</sup>/a and 12,809 Mt/a respectively from global rivers to oceans based on new and previous [29] data on Q and Q<sub>SS</sub> (1232 rivers for Q and 769 for Q<sub>SS</sub>), using Milliman and Farnsworth's method [29] to determine post-dam Q<sub>SS</sub> which takes account of regional change rate in sediment fluxes [30]. These results (Table S3 online) are very close to those obtained previously by Syed et al. [31] and Syvitski et al. [23,32]. At continental scale,
<figure><img src="image_10.png" /><figcaption>Fig. 4. The impact of two types of human activities on the most commonly occurring [Q, Q<sub>SS</sub>] trends under different climatic conditions.</figcaption></figure>

estimated values of annual Q and Q<sub>SS</sub> are similar to those in other studies [17,29,31–35], except for the values of Q and Q<sub>SS</sub> in Asia, Q in Africa and Q<sub>SS</sub> in Oceania (Table S3 online). These discrepancies arise because our updated data inherently take account of recent changes in Q and Q<sub>SS</sub> in Asia, Q in Africa, and we consider high Q<sub>SS</sub> values occurring in rivers located on ocean islands [29].

We have also assessed the total water and sediment fluxes from the land to oceans over the past 5–10 years, and compared it with the long-time average annual value. Based on the observed water flux in 828 large rivers discharging to the oceans, which represents 86% of the area of active water discharge [17], we find that Africa and Asia show significant decreases over the past 5–10 years when compared with the longer-term mean, while Eurasian Arctic and South America demonstrate increases. However, sediment flux to the sea declined significantly (by 1802–1909 Mt/a) in 193 large seaward rivers, representing 61% area of active water flux [17] (Table S4 online). Most of the reduction in land-ocean sediment flux occurs in Asian and African rivers, though small increases are found in Oceania and South America.

Carrying out a further extended study including smaller rivers in other 14%-39% of data-scarce active discharge areas, we estimate that the mean annual water and sediment fluxes to sea from the world's rivers decreased by 58–98 km³/a (0.25%) and 2617–2715 Mt/a (20.8%) respectively over the recent 5–10 five- to ten-year timescale, mainly due to the reduction in Q and Q<sub>SS</sub> that occurred in Asia (by 1.6%–2.0% in Q, and by 13.1%–13.2% in Q<sub>SS</sub>) and Africa (by 0.3%–0.4% in Q, and by 6.7%–6.8% in Q<sub>SS</sub>). The similar regional change rates of Q and Q<sub>SS</sub> obtained from 828 large seaward rivers are satisfactory approximations [30]. Looking at individual rivers it can be noted that the change in Q of the Amazon River,

which contributes globally about 18% of Q to the sea, is relatively small (1.0%–0.9%). In contrast, Q<sub>SS</sub> transported by the world's largest fluvial sediment carrier, the Yellow River, only reaches 11.3% of the 1950s level in the most recent decade, and its reduction from 1950s to 2000s accounts for 9.2% of global sediment flux to the oceans.

## 4. Conclusions

Based on the first exploration on the co-varying trends in water-sediment fluxes characterized with 9 typical binary patterns, we find that evolving patterns of change in the water and sediment fluxes of the world's large rivers are dominated by stationary behavior at the global scale, while 24% of the world's large rivers are experiencing significant changes in water flux and 40% in sediment flux. Furthermore, modification of the global hydrological cycle by climate change (in particular change of precipitation) and increased irrigation [36] are driving changes in water flux, while human activity may cause local decreases or increases in sediment flux through activity types A (in particular dam construction, irrigation and water-soil conservation) or B (notably deforestation, mining, and land clearance), respectively, thus altering the trends in Q<sub>SS</sub> in different river basins. More recent change identified by this study is the marked decrease (20.8%) in the global land-ocean sediment flux that occurred in the recent five to ten years, mostly (13.2%) contributed by Asian rivers due to accelerated human disturbance of river basin sediment budgets particularly as a result of dam construction. The findings presented highlight the increasing significance of climate change and human impacts on the world's large rivers which upscale to cause changes in global river fluxes of water, sediment and associated materials,
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 5.** Impacts of climate change and varying human activities on changing trends in water and sediment fluxes of global large rivers. (a) Contribution of precipitation variation to change in water discharge in large rivers (*P* < 0.05). (b) Contribution of precipitation variation to change in sediment flux in large rivers (*P* < 0.05). (c) DOR and Irrigation Index in the 1931 rivers with >40-year records of water discharge (0 are plotted as 0.01). (d) DOR and Irrigation Index in the 74 rivers with >40-year records of sediment flux (0 are plotted as 0.01).</figcaption></figure>

and important impacts on riverine, estuarine, coastal environments, and biogeochemical cycles.

**Declaration of Competing Interest**

The authors declare that they have no conflict of interest.

**Acknowledgments**

This work was supported by the National Natural Science Foundation of China (51721006 and 91647211).

**Author contributions**

Jinreni Ni and Desmond E. Walling designed the research. Li Li, Jinren Ni and Alistair G. L. Borthwick performed the research. Li Li,

Fang Chang and Yao Yue conducted the data analysis. Desmond E. Walling, Natalia Frolova, D Magritsky, Philippe Ciais, Yao Yue, Yichu Wang, and Chunmiao Zheng contributed new ideas and information; Li Li, Jinren Ni, Alistair G. L. Borthwick and Desmond E. Walling wrote the paper. All of the authors contributed to interpretation of the findings.

**Appendix A. Supplementary materials**

Supplementary data to this article can be found online at https://doi.org/10.1016/j.scib.2019.09.012.

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Li Li worked in Tianjin Institute of Water Transportation Engineering, Ministry of Transport. She received her B.E. in Port Channel and Coastal Engineering from Wuhan University in 2012 and Ph.D. in Environmental Engineering at Peking University in 2017. Her work focuses on water and sediment transport in rivers.

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Jinren Ni is currently a professor at Department of Environmental Engineering at Peking University. He graduated in Port Channel and Coastal Engineering from Wuhan University in 1982, received M.S. and Ph.D. in River and Coastal Dynamics and Hydraulics at Tsinghua University in 1985 and 1989, respectively. His work focuses material fluxes in aquatic ecosystems and river sustainability.