Global syndromes induced by changes in solutes of the world’s large rivers

Solute-induced river syndromes have grown in intensity in recent years. Here we investigate seven such river syndromes (salinization, mineralization, desalinization, acidification, alkalization, hardening, and softening) associated with global trends in major solutes (Ca2+, Mg2+, Na+, K+, SO42−, Cl−, HCO3−) and dissolved silica in the world’s large rivers (basin areas ≥ 1000 km2). A comprehensive dataset from 600 gauge stations in 149 large rivers reveals nine binary patterns of co-varying trends in runoff and solute concentration. Solute-induced river syndromes are associated with remarkable increases in total dissolved solids (68%), chloride (81%), sodium (86%) and sulfate (142%) fluxes from rivers to oceans worldwide. The syndromes are most prevalent in temperate regions (30~50°N and 30~40°S based on the available data) where severe rock weathering and active human interferences such as urbanization and agricultural irrigation are concentrated. This study highlights the urgency to protect river health from extreme changes in solute contents.

Previous studies of solutes and their changes are limited to either a few dissolved ions [8][9][10] or small spatial scales 3,11,12 . Gibbs 13 proposed that three major natural mechanisms control the chemistry of surface waters: atmospheric precipitation, rock weathering, and the evaporation-crystallization process. Edmond and Huh 14 suggested that bedrock lithology played a dominant role, and Berner and Kothavala 15 pointed out the importance of climate in chemical weathering. However, only about 17% of the present-day continental surface appeared free from direct human footprint 16 . Although acid deposition control measures reduced global freshwater Ca 2+ concentrations (even to levels approaching the threshold required by aquatic organisms) 8 , other human activities raised Na + , K + , SO 4 2− , HCO 3 − , and Cl − concentrations in rivers 2,6,11,[17][18][19] . Moreover, large dams have modified the solute content of rivers, as evidenced by the recent decreases in DSi in the Yangtze 20 and TDS in the Yellow River 21 .
Changes in solute fluxes caused by large-scale negative interactions between humans and the environment often result in socalled "global syndromes" once certain thresholds are exceeded 22,23 . In the early 2000s, Meybeck 16 defined river syndromes as negative feedbacks of river systems to human activities, primarily including river fragmentation, flow regulation, watersediment imbalance, and chemical and microbial contamination processes. Chen 7 investigated acidification, salinization, and alkalization in Chinese rivers, observing their association with coal consumption, agriculture, and hydraulic projects. Kopáček et al. 24 showed how regional socio-economic and land-use changes modified solute concentrations in the Vltava River, Czech Republic. More recently, Kaushal et al. 3 developed the concept of a freshwater salinization syndrome which links salinization and alkalinization processes along hydrologic flow paths from small watersheds to coastal waters. Using the long-term time series of specific conductance, pH, alkalinity, and base cation concentrations, Kaushal et al. investigated freshwater salinization syndromes in hundreds of stream and river sites throughout the continental United States 3 . Best 1 summarized typical anthropogenic stressors on the world's large rivers, which are of primary importance to varying solutes 8,25 and related river syndromes 26 .
Global trends in material fluxes through rivers not only provide estimates of riverine material budgets and changing terrestrial/ oceanic environments 9,27 in earth systems but also help to assess the status of river health 1,20,28 and identify anthropogenic drivers of such changes 11,29 . Based on a unique dataset of major solutes (Ca 2+ , Mg 2+ , Na + , K + , SO 4 2− , Cl − , HCO 3 − ) and dissolved silica derived from 600 gauge stations in 149 rivers (basin areas ≥1000 km 2 ), we proposed a framework of notable solute-induced river syndromes (salinization, mineralization, desalinization, acidification, alkalization, hardening, and softening) which could be identified using thresholds of solute concentrations and associated trends in the world's large rivers. Our study provides a dynamic perspective by which to determine the occurrence of solute-induced river syndromes and how they develop with covarying trends in runoff and solutes. This is of great significance in the maintenance of river ecosystem health.

Results and discussion
Co-varying trends in annual runoff (Q) and solute concentration (C) of the world's large rivers. As the primary flux transported by rivers, the trend in Q plays a crucial role in the evolution of solutes, e.g., through enrichment or dilution 3 . Figure 1 shows the co-varying trends of Q and TDS concentration (C TDS ) in global rivers. Data for TDS in 149 world's large rivers (Fig. 1a) and trend analysis for Q variation (Fig. 1b, statistically stable, decreasing, and increasing), revealed nine co-varying trends in Q and TDS, and their patterns of occurrence in each of six continents (Fig. 1c). For the 379 hydrologic stations (Fig. 1c), 83% maintained a stable trend in Q, dominated by the [Q, C TDS ] co-varying patterns I (stable C TDS trend, 59%), IV (decreasing C TDS trend, 12%), and VII (increasing C TDS trend, 12%), which mostly occurred in Asia and Europe. At these stations, the variation of C TDS is primarily controlled by changes in solutes themselves. The remaining six [Q, C TDS ] co-varying patterns were observed at only 32 stations (8.4%) scattered in Asia and Oceania where the C TDS variation is the combined consequence of changes in both solutes and Q.
At decadal scale, the [Q, C TDS ] co-varying trends may experience strong shifts due to changes in river dynamics typically caused by anthropogenic stressors 1,16 and so a viable approach is required to identify large rivers suffering anthropogenic syndromes. For example, the increasing occurrence of Pattern VIII in Asia is associated with widespread alarming trends in river discharge reduction and C TDS increase and is especially prominent at stations in the middle and lower Yellow River in China. As suspended sediment concentration in the Yellow River has decreased with time 30 , decoupling of TDS from the sediment trend has indicated the anthropogenic source and concentration effect 21 of increased C TDS . Pattern V, exhibiting decreasing trends in both Q and C TDS , mostly affects rivers in semi-arid regions of North America, including the Colorado and Rio Grande, and rivers in Oceania (such as Murray). Pattern VI, with decreasing C TDS and increasing Q, only occurs for the Gila River in North America and the Tapajós River in South America, indicating the dilution effect in small basins 31 . Finally, Pattern IX with increasing trends in both Q and C TDS is found to occur at a mere seven stations in North America, where severe anthropogenic perturbation is implied. Each of the nine [Q, C] covarying patterns of runoff and C TDS is described by a representative large river ( Supplementary Fig. 1); the underlying natural and human causes are summarized in Supplementary  Table 1.
The [Q, C] co-varying patterns were also characterized for dissolved solids (DS) ions and river discharge in the global large rivers ( Supplementary Fig. 2). Similar to the co-varying patterns of TDS, Patterns I, IV, and VII, each characterized by stable annual runoff, predominate (6.7-63%). Each solute ion exhibits distinct distributions of co-varying patterns across the six continents, reflecting differences in climatic, geological, hydrological, and human conditions. By examining the trends of individual DS ions, we identified several hotspots of anthropogenic stresses on global river systems ( Supplementary Fig. 3). For example, increasing trends of Ca 2+ , Mg 2+ , and Na + concentration and annual runoff in the Mississippi River ( Supplementary  Fig. 3a-c) correlate with freshwater salinization, mainly caused by human activities such as agricultural irrigation, salt pollution (e.g., road deicers, sewage, potash), mining, and increased use of easily weathered minerals (e.g., lime in agriculture and concrete in construction) 3,11 . Increasing K + and Cl − concentrations at the Toudaoguai station of the Yellow River ( Supplementary Fig. 3d, f) most likely resulted from saline return flows in the Qingtongxia and Hetao agricultural irrigation zones 32 . At Datong station of the Yangtze River, SO 4 2− concentrations increased with water discharge ( Supplementary Fig. 3e), owing to coal-burning and acid deposition in the basin 17,18 . In fact, the Chongqing-Guiyang region located in the upper Yangtze River basin is among the regions in the world most severely affected by acid deposition 18,19 . Surprisingly, the Ob River, one of the largest rivers in the Arctic zone, showed increasing HCO 3 − concentration and river discharge ( Supplementary Fig. 3g), driven by increased temperature, precipitation, permafrost thaw, agricultural liming, decreased acid deposition, and alteration of hydrology and vegetation 33,34 . With 22.9% of its basin covered by cropland and a population density of 9.51 people/km 2 , anthropogenic stresses are stronger in the Ob River basin than in any other Arctic river basin 35 . Finally, decreasing DSi concentrations at Lee's Ferry station downstream of the Glen Canyon Dam in the Colorado River ( Supplementary Fig. 3h) are attributed to decreasing silica-bearing sediment flux caused by the presence of the dam 20 .
Global fluxes of solutes to the oceans. Combining our dataset with others 36,37 based on the COastal Segmentation and its related CATchment (COSCAT) concept 38 , we estimated the global fluxes of solutes to the oceans (Fig. 2). Global TDS flux to oceans accounted for 6393 Mt/yr, about 68% greater than previous estimates of 3600-3800 Mt/yr 2,4,5 . Similarly, the estimated flux values increased significantly by 142% for SO 4 2-, 86% for Na + , 81% for Cl − , 71% for Mg 2+ , 62.5% for K + , 57% for Ca + , and 22% for HCO 3 − , compared to published data (Supplementary Table 2). The proximity of the DSi flux to previous estimates (within 10%) partially validated the present calculations because natural environmental variables determined the most stable DSi fluxes 2 . Moreover, the predominant patterns with stable Q covarying trends indicated that the accuracy of the estimated DS fluxes would be increased by using modeled discharge from Fekete et al. 36 . The increase in estimated global fluxes of other ions (especially SO 4 2− , Na + , and Cl − ) is likely to have been related to the use of data of improved quality, which included the effects of recent and long-term anthropogenic disturbances.  Supplementary Fig. 1h). The Amazon River (COSCAT-1104) exported only 473 Mt TDS annually to the Atlantic Ocean, even though annual runoff was extremely high. Moreover, the low TDS fluxes in the rivers of Africa ( Supplementary Fig. 1a) and Oceania ( Supplementary Fig. 1b) were due to stable TDS and low river discharge. Turning to specific ions, European rivers showed high fluxes of Na + and Cl − , as also found by Milliman and Farnsworth 2 , owing to intensive human activities (e.g., mining and agriculture). Magnesium fluxes were extremely high in the Indian Peninsula, South Asia (COSCAT-1337), resulting from the high magnesium content of water and soil in irrigation schemes, and leading to water quality deterioration and land degradation. In addition, high levels of bicarbonate fluxes were exported by Arctic rivers (COSCAT-1307, 1308, and 1309), as also observed by Drake et al. 33 .
Formulation and global distribution of solute-induced river syndromes. A solute-induced river syndrome represents a drastic change to a river system driven by extreme changes in dissolved solids concentration, pH, hardness, or alkalinity. This drastic change usually manifests itself as high/low contents of particular solutes, resulting from negative interactions between humans and the environment. Each solute-induced river syndrome, identified according to a certain threshold (see "Methods"), is characterized by specific symptoms, impacts, spatial distributions, and representative rivers (Table 1 and Fig. 3). In Supplementary Fig. 4, we further demonstrated the thresholds used to identify the soluteinduced river syndromes for given temporal trends in solutes concentration. Figure 4 presents the latitudinal distribution of mean annual DS concentrations under varying trends in Q. Stable, decreasing, and increasing trends in mean annual solute concentrations are identified in Supplementary Fig. 5. A previous study 10   Although chemical weathering is accelerated in geologically active and tropical regions, minimum solute concentrations occur close to the equator (5°S-5°N) owing to the already deeply weathered environment in Brazilian and African shields located in the central tropics 39 . Low concentrations could be further attributed to tectonic stability (>>100 My) and high precipitation in these wet tropical regions 40,41 . It appears that solute-induced river syndromes occur as combined consequences of changes in both solutes themselves and Q ( Fig. 4 and Supplementary Fig. 5), accompanied with data-driven trends at syndrome sites clustered in three "hot-belts" (30°-50°N, 30°-40°S, and 5°S-5°N).
Natural and anthropogenic impacts on changes in solutes with river syndromes. In 1970, Gibbs 13 proposed a diagram for interpreting the natural genesis of river water chemistry (atmospheric precipitation, rock weathering, and the evaporation-crystallization process). Figure 5a, b shows the Gibbs diagram derived from the dataset containing 60 gauge stations with solute-induced river syndromes, determined as average cation and anion ratios of dissolved ions (Ca 2+ , Na + , Cl − , and HCO 3 − ) plotted against TDS. Almost all the solute-induced river syndrome sites are scattered in the rock-weathering zone, except for several in the evaporation-crystallization and precipitation zones. As shown in Supplementary Fig. 6, sites with abundant precipitation that diluted solutes leading to decreasing C TDS trends lie in the precipitation zone. The majority of syndrome sites with increasing C TDS trends occur in the rock-weathering zone, indicating the possible influence of intense natural and anthropogenic impacts. Despite the notable variation trend of C TDS , the majority of sites affected by soluteinduced river syndromes exhibit a stable trend in Q (Fig. 5a, b), highlighting the importance of changes in solutes themselves. Conventionally, the Gibbs diagram has been used to explain natural drivers of river water composition solely for areas with low human interference. By incorporating the Q-C trends of solute-induced river syndrome sites in the world's large rivers, the present Gibbs diagram provides additional information potentially relevant to recent anthropogenic disturbance. For instance, irrigation intensifies rock chemical weathering 7,11 ; dam operations increase evaporation and humidity, altering the rainfall distribution while enhancing precipitation over the whole basin 42 ; and the urban heat island effect due to population growth and accelerated urbanization influences the global climate 43 . Increasingly, intense anthropogenic perturbation is therefore altering the natural processes that affect dissolved solids in rivers.
Lithology 12 , climate 15 , land use and land cover 26 , agricultural activities 7 , population 6 , and dam construction 25 are key environmental factors controlling solute concentrations. We, therefore, selected carbonate sedimentary and acid volcanic (or igneous) rocks, arid and temperate climate classification, global irrigation area, and urban land-use type, as primary influence factors for solutes, noting their significant levels from redundancy analysis (RDA) listed in Supplementary Tables 3-5. Extraction of these factors was based on the Global Lithological Map v1.0 44 , Köppen-Geiger climate classification 45 , Global Land Cover by National Mapping Organizations (GLCNMO) 46 , and Global Map of Irrigation Areas version 5 47 , as explained in Supplementary Table 6 and Supplementary Methods. Figure 5c, d depicts the latitudinal distribution (per 10°latitudinal belt) of temperate climate classification and urban land-cover classification. Supplementary Fig. 7 (per 5°latitudinal belt) Fig. 7a, b). Spatial variations in HCO 3 − and Ca 2+ concentrations relate to the carbonate sedimentary rock distribution 48 , whereas the DSi concentration correlates strongly with the percentage of acid volcanic rocks 48 . According to the Köppen-Geiger climate classification, the temperate climate that promotes optimum weathering is mainly distributed around 30-40°N and 30°S (Fig. 5c). Arid climate, also prominent around 30-40°N and 30°S ( Supplementary Fig. 7c), further strengthens anthropogenic impacts (e.g., in North America 49 ). The latitudinal distributions of irrigation (Supplementary Fig. 7d) and urbanized land cover (Fig. 5d) indicate severe anthropogenic interference 2,6 is occurring around 30-40°N, providing a partial interpretation of the maxima in Na + and Cl − solute ions in this region. Note that over half of both anthropogenic terrestrial and aquatic emissions occur between 20°N and 66°N 50 ; anthropogenic impact on dissolved solids should be significant in this region. RDA further differentiates natural and anthropogenic factors controlling solutes relevant to river syndromes (68 sites, Supplementary Fig. 8, and Supplementary Tables 3-5), particularly at 58 sites where syndromes occurred in the critical latitudinal belts (Fig. 5e, f). All natural factors displayed notable correlations with DSi (P < 0.05), except ice. Among the anthropogenic factors, irrigation presented the strongest correlation with the solutes considered (P < 0.05).
Arid climate correlated positively with Na + , SO 4 2− , and Cl − , but negatively with HCO 3 − and DSi (Fig. 5e and Supplementary Fig. 8a). Oppositely directed correlations are obtained between tropical climate types and the different solutes. Dilution of Na + , SO 4 2− , and Cl − ions would have occurred in areas of abundant precipitation, corresponding with Meybeck's findings 51 . In addition, rising river discharge could enhance dilution, leading to lower DS concentrations close to the equator (Fig. 4). In hot and humid regions, the HCO 3 − and DSi concentrations increase due to the rock-weatheringdominant sources. Temperate and cold (polar) climate types exhibit significant correlations with DSi, Na + , Cl − , or Ca 2+ , SO 4 2− , indicating the different weathering and dilution (or concentration) mechanisms for the solutes. For instance, the positive correlation of DSi with tropical/temperate climate and its negative correlation with cold (polar)/arid climates illustrates the greater proportion of siliciclastic sedimentary and igneous rocks in mid-latitude zones 44 , in accordance with Supplementary Fig. 7.
In view of the close relationships of sedimentary and igneous rocks with solutes ( Fig. 5e and Supplementary Fig. 8a), RDA was conducted for sub-classified rock types at sites with solute-induced river syndromes (Supplementary Fig. 9 and Supplementary Table 5). Carbonate sedimentary type correlated positively with Ca 2+ , SO 4 2− , and HCO 3 − , and negatively with Na + , Cl − , and DSi. Siliciclastic sedimentary type correlated positively with SO 4 2− , Na + and Cl − . Acid and basic volcanic types exhibited positive correlations with DSi and HCO 3 − , indicating that DSi was primarily derived from the weathering of igneous rocks. However, these two types of rocks presented negative correlations with SO 4 2− , suggesting that SO 4 2− was mainly derived from weathering of siliciclastic and carbonate sedimentary rocks containing sulfates.  Table 4) revealed the anthropogenic stresses due to agricultural activities. RDA plots obtained for anthropogenic factors ( Fig. 5f and Supplementary Fig. 8b) indicated positive correlations between Na + , Cl − and irrigation, agriculture, population, in accordance with findings from a previous study of the Mekong River 6 . Urbanization and degree of regulation (DOR) correlated positively with SO 4 2− and Na + , and negatively with HCO 3 − and DSi, implying that SO 4 2− and Na + may be effective indicators of the influence of urbanization and dam regulation. Here, increased SO 4 2− , Cl − , and Na + levels may be associated with severe anthropogenic disturbance (e.g., urbanization and irrigation), in agreement with other studies 2,3,32 ; whereas natural processes (e.g., weathering) played a vital role in controlling the concentrations of Ca 2+ , DSi, and HCO 3 − .
Sites experiencing solute-induced river syndromes in the three critical latitudinal belts exhibited the highest constrained variances (Fig. 5e, f), reflecting the intense impact of environmental factors on water quality. A holistic understanding of global solute-induced river syndromes is of fundamental importance for identifying and prioritizing regional management requirements and local amelioration strategies. The top priority is to offer advice regarding the time when a solute-induced river syndrome is likely to occur or has occurred. To facilitate practical management, a warning line could be set for each of the solute-induced river syndromes. Using a rule of thumb approach 7 , alarm values are identified in terms of the solute metrics as they approach 90% of the prescribed thresholds for soluteinduced river syndromes (Supplementary Fig. 4). The trends in solute concentration suggest the developing directions towards or away from the syndromes, while the co-varying trends in both runoff and solute concentration provide potentially important information on major causes of river syndromes associated with solutes evolution.
Given the significant impacts of various solute-induced river syndromes on water quality, contaminant transport, biodiversity, and ecosystem function (Table 1), precautionary countermeasures should be implemented for effective control of syndrome development in alarmed rivers. For instance, effective regulation of river flow, waste emissions, and agricultural runoff is necessary to control water hardening in North America, Western Europe, and South and East Asia. Improved management practices are required to prevent severe salinization in central and southeast North America and west Europe where population density is high. Knowledge of the inherent susceptibility of river systems to solute syndrome development should be utilized in regional decision-making processes, e.g., the presence of carbonated rocks encourages acidification, and semi-arid regions are very sensitive to salinization. Moreover, sustainable strategies should be made to alleviate climate influence on chemical weathering of rocks (particularly by HCO 3 − and DSi), with particular attention to mitigation of river syndromes in the process of urbanization and agricultural irrigation due to extreme changes in solute concentrations (especially Na + , SO 4 2− , and Cl − ).
The strict data requirements of simultaneous runoff (Q) and solute concentrations (C) limited the data available for this study. Considering the potential influence of the location of monitoring sites on the spatial distribution of solute-induced river syndromes, future effort should be made to enrich the dataset with greater coverage through enhanced long-term monitoring, particularly for rivers in areas with scarce data. To compensate for the lack of available data, biogeochemical predictive models should be developed in conjunction with global monitoring campaigns, to link the solute-induced river syndromes with the natural conditions and human disturbances of the basins, and predict their future evolution under different scenarios.
In summary, using a newly established global database containing annual runoff and solute concentration information at 600 stations in 149 large rivers, we examined the geographical spread of seven solute-induced river syndromes (salinization, mineralization, desalinization, acidification, alkalization, hardening, and softening) in the world's large rivers. In general, a stable [Q, C] trend predominated the nine representative co-varying trends across the global rivers. However, significant increases occurred in total dissolved solids (68%), chloride (81%), sodium (86%), and sulfate (142%) fluxes from rivers to oceans worldwide, leading to typical solute-induced river syndromes on exceeding specific thresholds. Our study revealed that the syndromes were mostly concentrated in two latitude zones (about 30-50°N and 30-40°S, respectively) which experience severe rock weathering accelerated by anthropogenic disturbances such as rapid urbanization and agricultural irrigation. Despite acidification being observed close to the equator (5°N-5°S), such syndromes were mainly exacerbated by extreme dilution of solutes vital for river health, such as HCO 3 − in the rivers of South America. This study provides insight into global changes in major river solutes and highlights their importance to the geochemical cycle, and is of significance to the water security management of large river ecosystems.  are mostly located in North America (54.8%), followed by Asia (14.8%), Oceania (12.2%), Europe (8.2%), South America (7.5%), and Africa (2.5%) (Fig. 1). The 149 rivers collectively drain 46 million km 2 of watersheds. For dissolved solids, 64-73% of the stations have a record length no less than 10 years (Supplementary Table 9) with the longest record up to 93 years. Data quality was ensured by detecting and removing 1888 outliers (2.3% of all data) in the time series of solutes. In a subset of our data (6124 samples with available concentrations for all seven charged ions of interest during the same period at the same monitoring station), 89% of the data points met the normalized inorganic charge balance criteria (−10%~10%) for cations and anions, guaranteeing the data were of high quality. The Supplementary Methods provides a detailed description of the database compilation and quality assurance procedures.

Methods
Estimation of solutes fluxes. A hybrid approach was used to calculate the annual fluxes of solutes. When the measured annual average concentration of a specific solute was available for a given year at a specific station, the annual flux was determined as the product of the annual average concentration with annual runoff. When measured data were unavailable, the LOAD ESTimator (LOADEST) model 52 was used to estimate annual average concentrations and loads for the available daily or monthly river flow. For 15,702 available data points, the observed and calculated annual fluxes were in good agreement (R 2 > 0.9, Supplementary  Fig. 10), suggesting the reliability of the hybrid approach that exploited both observed and predicted datasets. The COastal Segmentation and CATchment (COSCAT) 38 concept was used to estimate the fluxes of solutes to the sea based on average data calculated from our datasets, ion concentration datasets by Meybeck and Ragu 37 , and the estimated discharge of each COSCAT 36  Determination method for global solute-induced river syndromes. Three solute-induced river syndromes were identified following Meybeck's classification 51 for different solute trends: salinization with Σ + (Σ + = C Ca 2+ + C Mg 2+ + C Na + + C K + ) > 24 meq/L indicating an increasing trend, mineralization with Σ + > 3 meq/L indicating an increasing trend, and desalinization with Σ + < 1.5 meq/L indicating a decreasing trend. To determine whether a river experiences acidification or alkalization, both pH and alkalinity (or acidity) must be considered 18,21,33 . Here, we calculated the ratio of hardness to alkalinity, and extracted mean pH values for rivers from the GEMS database 55 . A hardness to alkalinity ratio >1 suggested an acid input from anthropogenic sources 56 . The acidification syndrome was deemed to occur in a river when pH <7 and the ratio of hardness to alkalinity >1 indicating an increasing trend. The converse indicated an alkalinity syndrome. The hardening syndrome in a river corresponds to an increasing trend in hardness >120 mg/L, whereas the softening syndrome corresponds to a decreasing trend in hardness <60 mg/L. The Supplementary Methods give further details of the methodology used to identify the different solute-induced river syndromes.
Extraction of typical natural and anthropogenic environmental factors. Lithology 44 (sedimentary, volcanic and metamorphic rocks, and ice) and Köppen-Geiger climate classification 45 (reclassification of arid, temperate, tropical, cold, and polar climates) were selected as natural factors. Primary anthropogenic environmental factors were land cover (reclassification of vegetation, urban, agriculture, and bare land), irrigation, dam, and population. The Supplementary Methods describe the approach used to extract information on the environmental variables from the metadata; Supplementary Table 6 lists the detailed environmental information.
Reporting summary. Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Data availability
Data sources for concentrations of dissolved ions and runoff in the world's large rivers are available within the paper and its supplementary information file. Data on solutes concentrations (C), annual runoff (Q), the co-varying trend in Q and C, and the identified solute-induced river syndrome if applicable, at each river station have been deposited on figshare [https://doi.org/10.6084/m9.figshare.14910399] 57 . Source data are provided with this paper.