key: cord-0955744-di865dw7
authors: Best, Jim; Darby, Stephen E.
title: The Pace of Human-Induced Change in Large Rivers: Stresses, Resilience, and Vulnerability to Extreme Events
date: 2020-06-19
journal: One Earth
DOI: 10.1016/j.oneear.2020.05.021
sha: 7aaaa4e9a4504749a24b3b32961ade7246a3212d
doc_id: 955744
cord_uid: di865dw7

The world’s great rivers are threatened by a range of anthropogenic stresses—of which climate change is just one—that decrease resilience and increase vulnerability to extreme events. Future governance must recognize both the rate of change associated with these stressors and the potential for extreme events to transgress sustainability thresholds.

The world's great rivers are threatened by a range of anthropogenic stresses-of which climate change is just one-that decrease resilience and increase vulnerability to extreme events. Future governance must recognize both the rate of change associated with these stressors and the potential for extreme events to transgress sustainability thresholds.

The world's large rivers are changing fast. As home to over three billion people and harboring some of the planet's most diverse ecosystems, large rivers are hotspots of resources, agriculture, trade, and energy production. Many large rivers flow through developing nations, where much of the population is vulnerable to environmental and ecological stresses. Especially in areas near the poverty boundary, both subsistence and cash elements of the economy tend to rely disproportionately on the ecosystem services associated with the water and nutrient fluxes delivered by large rivers to floodplains and deltas. However, the demands posed by burgeoning population growth are placing unprecedented stresses on the world's great rivers; 1 without urgent interventions, some face ecosystem collapse in the coming decades. 2 These anthropogenic stressors operate on a range of timescales, and we argue that their effects potentially amplify the risks posed by extreme climate events 3 and thereby increase the likelihood that key resilience thresholds will be crossed. Moreover, the strong link between ecosystem services and livelihoods (associated with the unifying nexus of water, energy, and food security 4 ) means that, along many large rivers (such as the Ganges, Mekong, and Nile), these stressors are inhibiting efforts to meet the UN Sustainable Development Goals (SDGs) by 2030.

Rivers respond to disturbances, such as changes in water or sediment flux, through self-adjusting processes of erosion and sedimentation. These responses typically involve feedbacks that impart some resilience, allowing rivers to absorb a degree of change. Climate change, as manifested through a complex global pattern of future floods and droughts, 5 presents a background stress that is increasing through time and pushing this flexibility to its limits ( Figure 1A ). River resilience can further be lowered by a range of other anthropogenic stressors that could present slow ongoing changes or extreme events that operate over short timescales ( Figure 1A ; Figure 1B , E1 and E2). For example, the ways in which river channels are engineered 6 could cause increased extremes in floods and droughts. This reduction in resilience makes river systems more vulnerable to the increasing magnitude and frequency of such extremes, which increases the likelihood that the tipping point for system resilience will be crossed ( Figure 1A ). Such changes can have immediate consequences, such as hazardous riverbank erosion, which damages infrastructure and threatens lives, but could also impose a legacy of change that persists for centuries.

The array of anthropogenic stressors, beyond climate change, that threaten big rivers 1 differ in both the rapidity of their effects ( Figure 1B ) and the extent to which they leave a legacy as river responses to disturbance translate upstream and/or downstream.

Changing river flows also modulate the movement of sediment to floodplains and deltas. Timescales of effects for climaterelated changes in river flows, and their extremes, range from decades to hundreds of years. In addition, land-use changes, such as deforestation or reforestation of catchment hillslopes and floodplains, can modify the quantity of water and sediment entering rivers through soil and bank erosion over timescales of decades to centuries ( Figure 1B) .

The construction of mega-dams for hydropower, flood control, irrigation, and water supply is booming such that some 3,700 dams are planned or under construction and, if fully implemented, they would decrease the number of remaining free-flowing rivers by 21%. 7 Dams trap sediment, alter flow regimes, trigger riverbed incision and bank instability, and in tropical regions, cause substantial release of the potent greenhouse gas (GHG) methane 1 as a result of vegetation decay. Such effects are compounded by water diversions and the interlinking of rivers within, and between, river basins. 1 Planning and constructing these schemes take decades ( Figure 1B ), but once they are built, their consequences on riverine ecology (e.g., fish and mammal migration once a reservoir begins to fill) can be rapid (months to years). Where the world's great rivers enter the oceans, they form deltas that are home to over half a billion people, often in sprawling megacities, as well as support a rich and essential agriculture. These deltas are prone not only to longer-term sea-level rise linked to GHG-induced warming but also to shorter-term (several years to a decade) stressors such as sediment starvation due to upstream sediment trapping and extraction, as well as subsidence that is often enhanced by groundwater extraction 8 ( Figure 1B) .

Other anthropogenic stressors occur over just months to years ( Figure 1B ). Sand and gravel mining are destroying riverbeds across the globe as the demand for concrete and silica escalates 9 and have been linked to severe ecosystem degradation, illegal mining, organized crime, and social injustice. The introduc-tion of non-native species-such as fish, invertebrates, and vegetation-can have almost immediate ecological effects and economic consequences, potentially compounded by river interlinking. Studies of the River Rhine reveal dispersal rates for some non-native aquatic species of up to 461 km per year, and combatting the spread of Asian Carp within the Mississippi River and US Great Lakes has been speculated to cost up to $1.5 billion USD over the next decade. Pollution from industrial, domestic, and agricultural sources could pose a near instantaneous threat. Alarming new research has revealed the extent of riverine antibiotic pollution, 10 introduced into watercourses through human and animal waste and leaks from wastewater treatment plants and chemical factories. Such potent antibiotic pollution is present worldwide and aids the development of antimicrobial resistance. 10 All of these anthropogenically induced stresses must be viewed in the light of shocks generated by political, social, and economic extremes. The current coronavirus disease 2019 (COVID- 19) pandemic is already influencing regulatory frameworks: the US government has eased its enforcement of pollution monitoring, enabling polluters to avoid penalties if they argue that violations are related to the COVID-19 crisis. 11 Political and societal shocks, such as that imposed by the First Gulf War, had major consequences for river pollution and water resources in the Tigris-Euphrates river basin, a situation compounded by upstream damming in Turkey.

The Mekong River is an exemplar of the challenges faced by many large rivers ( Figure 2) . Identified by the Intergovernmental Panel on Climate Change (IPCC) as one of the three most vulnerable deltas in the world, the Mekong Delta is home to 18 million people, most of whom are highly reliant on agriculture for their livelihoods. Complex, multi-faceted, and rapid anthropogenic processes are contributing to delta drowning and salinity intrusion, stressors that demand immediate attention. Land subsidence, caused by groundwater pumping, has in the last 25 years exceeded rates of GHG-driven sea-level rise by an order of magnitude, 12 and projections 13 indicate that, if unmitigated, this process alone will submerge the delta below the current sea level by the end of the 21 st century ( Figure 2B ). Groundwater-induced subsidence has also led to arsenic pollution, 12 and arsenic mobility has been further enhanced by seasonal wetting and drying and saltwater intrusion.

A further key stressor is sediment starvation due to declining river sediment loads caused by (1) the changing frequency of extreme events (tropical cyclones; Figure 2 , main panel), 16 (2) sediment trapping behind dams ( Figure 2C ), 1,2,15 and (3) large-scale mining of river sand (Figure 2 , main panel). 17 The combined impacts are so large that riverbed levels have lowered by ca. 2-3 m in only the last decade, destabilizing river banks 17 and enabling saltwater incursion at high tide to propagate many kilometers further inland, posing a severe threat to agricultural production. The near-instantaneous impact of sediment starvation is thus an order of magnitude more significant as a driver of saline intrusion than ongoing, climate-driven sea-level rise.

The contribution of climate change to the processes of delta ''drowning'' and saline water intrusion is thus best viewed as a slow-onset hazard that progressively increases system risk ( Figure 1A) . Meanwhile, multiple other anthropogenic stressors (Figure 2 ) reduce the resilience threshold of the river and delta, increasing the likelihood that extreme events will cross this critical state ( Figure 1A, T1) . It is thus imperative to consider the timescale and magnitude of these stressors that, when combined with long-term, progressive reductions in resilience, threaten to force the Mekong Delta into a new state, as reflected in the rapid (about one million people in the last decade) rates of human migration from the region. These challenges are exemplified currently by severe drought in the delta region, which is experiencing record-low water levels. This drought is undoubtedly partly driven by belowaverage levels of precipitation in the Lower Mekong Basin, but it has also been speculated that extreme levels of water storage behind dams in the Upper Basin in China have amplified the impacts. 19 One estimate suggests that the drought has caused the fish catch from the Tonlé Sap Lake in Cambodia, on which three million people depend for their main source of protein, to decline by 90%.

The challenges faced by the Mekong River, a transboundary river crossing through six countries (Figure 2) , and its delta are compounded by issues of governance, particularly a lack of international and interprovincial coordination over water use. Decision making is complicated by the diverse interests among, and between, provinces as to which sectors and farming practices should be pursued. This includes competition for water between shrimp farming and agriculture, diverse rice-farming practices, and a lack of communication between policymaking and community levels, resulting in frequent implementation problems, particularly concerning drought and flood policies.

What Is Needed? Consideration of anthropogenic stressors and extreme events tells us three things about the future of the world's large rivers.

First, the timescales over which stressors, including but also beyond The Mekong Delta is at risk of drowning in the coming decades as a result of a combination of climatedriven sea-level rise 14 (A), anthropogenic subsidence 13 (B), and sediment starvation 2,15 (C), which prevents sediment deposition offsetting a rising sea level. Sediment starvation is driven by shifts in climate extremes (tropical cyclone tracks; 16 main panel) along with a combination of sediment trapping 2,15 behind hydropower dams (squares; main panel and inset C for timeline) and intense sand mining 17,18 (orange circles; main panel). Groundwater pumping is also linked to arsenic contamination in the delta's aquifers 12 (see inset As; 10 mg L À1 is the World Health Organization drinking-water standard 12 ). climate change, exert an effect must be recognized and incorporated into management responses. Some stressors can act quickly and pose a more immediate threat to the integrity of many riverine ecosystems.

Second, such stressors are frequently compounded, making it more likely that resilience thresholds will be approached or crossed, particularly when subjected to extreme events ( Figure 1A) .

Third and lastly, the changes to some large rivers as a result of non-climaterelated pressures are now so great and so rapid that there is a clear danger of imminent, and irreparable, environmental change. Ironically, this affords some grounds for optimism. Just as river response to some anthropogenic disturbances is rapid, when effective interventions are made, a return to a safer trajectory can be commensurately swift. For example, a range of measures along the Chang Jiang (Yangtze) River, including dredging plastic, relocating factories, and banning waste discharge and single-use plastic goods, are having major effects, 20 potentially reducing plastic pollution into the oceans.

Any strategy for enhancing resilience and safeguarding the critical services that large rivers provide must recognize the specific characteristics and challenges posed in their management. Often-but not always-these rivers flow through multiple jurisdictions. Sometimes-but not always-they are in developing nations. The different time lags in river response, as well as their legacy over individual and generational time spans, demand integrated governance as an essential prerequisite in any effort to sustain rivers.

Effective international institutions must provide a channel for scientific advice and help support and enable the capability and capacity of local, national, and transnational river-management organizations. Centrally, river governance ( Figure 1 ) must include local stakeholders and incorporate issues of social equity, inclusivity, and gender. The 2011 Vienna Declaration on the ''Status and Future of the World's Large Rivers'' called for such a UNESCO-led initiative, but thus far effective progress has been slow. Much as the IPCC was created by the UN to assess the state of knowledge on climate change, its social and economic impacts, and potential response strategies, it is overdue that a similar UN International Panel for the World's Large Rivers be established and charged with a similar holistic brief. Indeed, the effectiveness of IPCC recommendations will be limited for billions of people living within large river basins unless the presence, timescales of effect, and nature of other anthropogenic stressors and extremes are incorporated into policy. Creative financial instruments are also essential to delivering the investment necessary to fund restoration, protection, and management. Carbon credits have, not without controversy, created incentives for polluters to reduce or offset their carbon emissions. Analogous schemes could, over time, create a significant market to fund river restoration and protection efforts worldwide.

Some of the world's largest and most productive rivers are now at a critical juncture. It is our choice, and entirely within our control, as to whether sustainable futures-and the essential riverine contributions to achieving the UN SDGs-will be attained or foregone. The effects of these stressors, and critically their timescales, must be incorporated into planning. Progress toward environmental sustainability and food security within these regions hinges on our response over the next decade. If we fail to recognize the timescales of change, incorporate the effects of extremes, and implement an appropriate international response, in 50 years' time our despondent descendants will be baffled as to why.

The seeds for this paper were sown when J.B. received a Diamond Jubilee International Visiting Research Fellowship from the University of Southampton, for which we are very grateful. S.E.D.'s contribution to this work was supported, in part, by award NE/S002847/1 from the UK Natural Environment Research Council. We would like to thank Chris Simpson for his skillful preparation of the artwork.

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