Asia’s rivers affect more than half of the world’s population, providing water for cities, agriculture, transportation, and power generation and contributing to flooding and landslide hazards. These rivers also play important roles in many physical and biogeochemical processes on Earth’s surface, shaping the landscape and conveying huge quantities of water, sediment, and dissolved constituents to marginal seas—the regions that separate coastal zones from the open ocean.
“Mega rivers” in Asia—considered here to be those with historical annual sediment discharge of about 100 megatons per year or greater—share a common source in the Himalaya and Tibetan Plateau region (Figure 1), but a wide range of human and natural factors influences their fates. In many of these long, large river systems and their receiving basins, human activity (e.g., dam construction, agriculture, river management, trawling) has dramatically changed the length of time that particulate and dissolved matter spends in the river on its journey to the ocean.
Of particular concern in these rivers is human alteration of the transit and sequestration of water, sediment, and bioactive elements like carbon, iron, sulfur, phosphorus, nitrogen, potassium, and silicon. Anthropogenic changes to these fluxes affect the sustainability of deltas and coastal oceans, in turn imposing profound consequences on society that have only recently begun to be appreciated. We must understand the ongoing changes in fluxes and fates for river-derived materials to understand current impacts and predict future trends concerning such socially relevant issues as the global carbon budget and ocean acidification, eutrophication, pollution, and coastal erosion.
In October 2019, scientists gathered in Xiamen, China, for a workshop on the Asian sedimentary continuum (ASC). The term sedimentary continuum, in this context, is intended to reflect the myriad transport pathways of solids and dissolved matter associated with rivers, from the upland source regions through tidal rivers to the ocean’s receiving basins. The workshop participants reached a consensus on the need to develop an international collaborative program to explore how human-induced landscape and seascape alterations and climate change affect the sedimentary continuum and the cycling of bioactive elements from mountains to the deep sea along major Asian rivers.
Different Paths from the Plateau
The Asian mega rivers flow through an enormous range of geologic, climatic, and land use conditions, providing opportunities to study rivers that have common origins but vastly different downstream environments. Inputs of water, along with mineral and organic matter from the Tibetan Plateau (sometimes called “Asia’s water tower”) and the nearby Himalaya, spawn the mega river systems that together convey approximately 25% of the world’s riverine sediment flux to marginal seas.
The catchments and receiving basins of these rivers have distinctive physical settings, and humans influence them in vastly different ways. For example, sediment fluxes through many of these rivers have whipsawed because of human activity during the Holocene. Early agricultural and other land use practices resulted in a large increase in sediments in the rivers. Then, beginning in the 1950s, accelerated dam construction caused a dramatic decrease in sediment loads. This latter trend has been particularly acute for Chinese rivers, where sediment supplies have declined to about 20% of predam levels during the past half century. During this period, about 250 “mega dams” and many smaller dams were constructed across Asia [Gupta et al., 2012].
In contrast, the large rivers that drain to the ocean via Bangladesh and Myanmar have not yet seen such dramatic decreases—the Ayeyarwady (Irrawaddy) and the Thanlwin (Salween), both of which flow through Myanmar, are the last remaining free-flowing rivers in Asia [Grill et al., 2019].
Despite the strong societal relevance, temporal and spatial variations in the compositions and concentrations of particulate and dissolved matter in these rivers are poorly known. Likewise, the effects of nutrients and pollutants supplied from Asian rivers to the coastal ocean are not thoroughly characterized. Such effects include hypoxia and acidification, especially under rapid climate change and enhanced human activities. Contrasting the journeys of materials through two or more Asian rivers with a common origin would allow scientists to explore major controls on sediment fate and bioactive element cycling along their sedimentary continuums.
Reading the Rivers’ Changing Signals
The transfer and cycling of dissolved and particulate material in rivers are complex and, in many systems, have been strongly perturbed by human activity [e.g., Syvitski and Kettner, 2011]. We need to understand the mechanisms by which sediment and associated bioactive constituents (the environmental signals) vary in space and time along the sedimentary continuum to determine economic and societal impacts related to current and future human perturbations.
Climatic and geologic influences like precipitation patterns and rock weathering significantly influence the particulate and dissolved loads entering rivers. Once these materials become part of river flows, river systems process and preserve source input signals in different ways depending in part on prevailing climates, which can range from temperate to tropical, and on the temporary storage and release of materials along the sedimentary continuum, among many other factors. After the rivers enter the coastal oceans, sediment continues to be processed, and chemical exchanges occur as a function of oceanographic conditions and other human influences.
One important variable in understanding biogeochemical cycling of river-borne material is the residence time of sediment in different parts of the sedimentary continuum. For example, depositing sediment in natural river floodplains temporarily removes material from the transport system. On floodplains, this material is exposed to weathering and diagenetic (sedimentary rock–forming) conditions that may affect the breakdown of mineral and organic constituents and the release of solutes into groundwater. In a natural state, the same sediments once deposited onto a floodplain may be reentrained into the transport system as a river switches course. Thus, a mixture of new and “aged” material dictates the overall composition of solid and dissolved materials in rivers.
As rivers and floodplains become extensively modified by human activities, the amount of time that sediments spend in the river and floodplain can be altered in ways that in turn disrupt natural sedimentary and biogeochemical cycles. Increased erosion and runoff from agricultural practices can decrease the time that sediments spend in a floodplain. In contrast, if human activities disconnect rivers from their floodplains, for example, with flood control structures, sediment residence times in floodplains can increase.
Similar examples of human influence can also be found in the marine portion of the sedimentary continuum. This area is often assumed to be mainly a sink where sediments are buried offshore, effectively isolating them from further exchange with overlying ocean waters. However, even in these offshore areas, human activities may play major roles in the exchange of bioactive elements between the seafloor and ocean waters above. For example, bottom trawling may affect nearly 100% of the continental shelf seafloor in heavily fished regions [Puig et al., 2012]. This activity churns the seabed, injecting oxygen-rich water into the sediment, dramatically changing the conditions for seafloor diagenesis and the flow of dissolved constituents into the coastal marine waters above, and ultimately altering chemical exchange between the sediments and the wider ocean.
Building an Integrated View
Over the past several decades, the geoscience community extensively investigated Earth surface processes along many Asian continental margins, but most studies have looked at individual components of the system. As a consequence of this compartmentalized approach, fundamental questions about the overall fluxes and fates of material in Asian mega rivers remain unanswered. Changes effected by human disturbances along rivers are exacerbated by climate change (including monsoon conditions) and sea level rise, and they represent clear and imminent threats to human health and economic development. Yet the future magnitude and trajectory of such impacts are largely unknown.
For example, the planned construction of major dams on the Ayeyarwady and Thanlwin Rivers, which flow into the northern Andaman Sea, likely will have major consequences for material transport, coastal erosion, and land loss. Like many densely populated major river deltas, the low-lying areas of the Ayeyarwady delta are considered to be under high threat from accelerated sea level rise. Although the Ayeyarwady and Thanlwin are currently free-flowing, even now they are not free of human influences. Sand mining in the rivers and deforestation of coastal mangroves are accelerating, potentially driving significant shoreline erosion and land loss.
Another example is the Yangtze River, considered one of the “mother rivers” of China, which extends thousands of kilometers from the Tibetan Plateau to the East China Sea. Inputs from this river into the sea mix with the large cumulative inputs from the small mountainous rivers of Taiwan delivered to the Taiwan Strait, which connects the East and South China Seas.
In contrast to large river systems in North America and many other parts of the world, which have generally experienced human influences for hundreds of years or less, humans have been altering sediment loads in Asian mega rivers for more than 1,000 years [Walling, 2011]. Today, human influences on the Yangtze are particularly acute, with many megacities (e.g., Chongqing, Shanghai), large dams (e.g., Three Gorges), and areas of dense industrialization lining its course and polluting its waters. The Yangtze supplies more plastic to the ocean than any other river [Lebreton et al., 2017] and has experienced dramatic climate, land use, and dam-induced fluctuations of its water and sediment fluxes [e.g., Yang et al., 2015].
Thus, a large-scale interdisciplinary program is needed to address key questions regarding the ways that human, climate, and sea level changes will affect public health and economic development in the future in such complex systems.
A Transformative Program
Future progress for the Asian mega river research community presents challenges as well as opportunities. A key to success will be the continued development of predictive models that incorporate climate change; landscape evolution; sea level change; and the storage, transfer, and transformation of materials through space and time. We also need to better understand the coupling between particle transport processes and the biogeochemical signals they carry from the terrestrial to the marine environment.
The program proposed at the October 2019 workshop would be transformative, in that it would foster research that transcends traditional disciplinary boundaries and considers the sedimentary continuum holistically. Scientific communities (representing, e.g., geology and biogeochemistry) have a rich opportunity to collaborate and provide a comprehensive understanding of sedimentary dynamics and bioactive element cycling by investigating residence times, transport pathways, and exchanges of sediment and associated organics.
Providing synoptic measurements along the entire sedimentary continuum and developing the ability to predict future changes, from mountains to the deep sea, will require innovations in several areas:
- in the development of new satellite sensors, such as those aboard NASA’s Surface Water and Ocean Topography (SWOT) satellite
- in modeling methods that couple landscape and seascape evolution and biogeochemical and sediment transport modules
- in observational approaches such as cabled observatories, gliders, and benthic tripods
- in geochemical proxies using, for example, nontraditional stable or metal isotopes or novel biogeochemical markers
These approaches must be combined with insights gleaned from the sedimentary record to understand past conditions and with modeling approaches that allow us to predict future trends. In this way, a new understanding of the sedimentary continuum along Asia’s mega rivers through space and time can be applied to address health, environmental, and economic concerns of the future.