Arteries of the Earth: Why Rivers Run Everything

TUESDAY, 20 JULY 2021

Less than one percent of rivers in England and Wales are free of man-made barriers. Such is the extent to which our nation is built around running courses of fresh water. The story around the planet is no different — Europeans have removed 90 percent of their original wetlands and floodplains, which are crucial parts of river ecosystems. A study published in the journal Nature as far back as 2010 estimated that human impacts threaten the quality of rivers that serve 80 percent of the world’s population.

Not only do rivers play a vital role in our economies and everyday lives today — fertilising floodplains and providing clean hydro-electric power (HEP) to cities and rural communities — they also represent a fundamental control on the global carbon cycle, regulating climate feedbacks on 100,000-year timescales. The role of rivers has developed and changed through the evolution of our planet, but rivers have always played a critical role in shaping Earth’s surface and the life that takes place there. Rivers truly are the arteries of the Earth.

In this issue’s FOCUS piece, BlueSci takes a look at different ways in which scientists — within Cambridge and outside — study rivers to tell us about the world we live in and underscore their importance to our civilisation. First, we explore the co-evolution of rivers with our planet across billions of years, looking at how they shaped the different ecosystems for life across its entire evolutionary history. In section two, we discuss rivers’ vital role in the cycling of Earth’s crust, which has kept Earth’s climate habitable for hundreds of millions of years, and how we can use modern rivers to understand this. Finally, we look at the importance of rivers to the human communities who rely on them — taking a focussed look at sand mining and damming in south east Asia. These are just some of the ways scientists observe rivers, giving us fascinating insight into the power of studying just one small facet of our incredibly complex planet Earth.

Rivers in Deep Time

Here’s an interesting question: what is the oldest (extant) river on Earth? The answer is, as it turns out, contentious. The geological methods for determining the age of rivers can be tricky, and there is also debate about how far a river can be considered the same river over time if, for example, it veers too far off course. The Mississippi is thought to have existed in one form or another, draining the North American continent for 300 million years. But the prize may go to Australia’s River Finke, whose course cuts right through mountains known to be over 350 million years old. This puts its age as Devonian — the name geologists give to the period between 419 and 359 million years ago.

Imagine the history this river has seen; the Devonian hosted the first serious radiations of both plant and animal life onto dry land, leaving the water for the first time. It has seen Pangaea rise and fall. The world in which that river was born is one fundamentally different to our own.

Indeed, how rivers themselves have changed over time is an area of active research in the Earth Sciences. The Devonian period was actually a critical interval for rivers on Earth. Before the Devonian, most rivers on Earth were sheet-like and braided. Picture wide, unconstrained streams flowing over sandy plains stretching hundreds of kilometres across. Today, some of the closest analogues to pre-Devonian rivers are found not on Earth but in the three-billion year old dried-up stream beds etched into the surface of Mars. In the Devonian, the evolution and spread of deeply rooting, vascular land plants saw river banks stabilised, becoming resistant to the erosive forces that had caused their previous unconstrained style. Plant growth on river banks also baffled flooding fluids, trapping mud and building up established floodplains and levees. This was the beginning of the dominance of meandering rivers. Today, almost all rivers on Earth are meandering for a major part of their course, and so it has been since the Devonian.

What effects could such a drastic change in the flow of freshwater on a global scale oversee? For one thing, changing fossils found in preserved river systems reveals the development and expansion of vibrant new ecosystems. Trees develop in the late Devonian, and their subsequent evolution lead to logs falling into and being transported by streams — causing the Earth’s first log jams — and piling up in anoxic swamps, eventually giving rise to the world’s first ‘coal age’. Coal seams from this era form most anthracite and bituminous coals mined today.

Perhaps most important though is that trapped mud mentioned earlier — the mud trapped in river systems after the evolution of their meandering, floodplain-bearing character, and also the mud created by the chemical action of the land plants colonising the river banks. These muds are made up mainly of a group of minerals called silicates. When silicates are transported to the ocean, they play a key part in reactions that can draw down atmospheric carbon dioxide (CO₂). That means the advent of land plants and their impact on rivers — in this sense, silicate-routing systems — could have a fundamental control on atmospheric CO₂ levels. This issue is at the cutting edge of geological research today.

An interview with Dr William McMahon

Post-doctoral research associate in the Department of Earth Sciences at Cambridge, who studies environments and ecosystems on the early earth

STH: What to you is the importance of studying ancient rivers?

WJM: I think what people don’t appreciate so much is the significance vegetation makes up today as a percentage of global biomass. There was a study published in PNAS in 2018 that estimated that if you added all the biomass of all the different forms of life on Earth, 87% of it is plant biomass. Plants are in effect earth's most powerful ecosystem engineer, they really dictate all sedimentary processes on earth — be it in the river systems which they directly affect, or downstream into the marine realm where there are no plants but the effects of plants upstream have been so profoundly felt that they have a serious knock-on effect. Everything we know about how river systems behave at the present day is fundamentally tied to how we know plants influence river systems. You can't study rivers at the modern day and not consider vegetation effects. But what geologists need to appreciate, is that for 90% of Earth history, that was not true. Plants are a relatively recent intervention. They caused a profound biological change, possibly the most profound biological change Earth ever underwent, and it happened in just a 100-million-year interval [amidst 4,500 million years of Earth history]. We then see a couple of key transitions, relating to mud production and mud retention on floodplains.

STH: And this is important for the global carbon cycle.

WJM: Yes — plants decrease the rate at which mud is moved from the land to the oceans, by trapping it on river floodplains, and might even increase the amount of mud produced, by chemical weathering. The global carbon cycle depends on the functioning of the entire ‘Earth circuit’, from eroding mountains all the way through rivers into oceans, then subduction [rocks being forced back inside the earth and melted down at boundaries between tectonic plates]. If you tamper with any part of that system, it's going to have a knock-on effect on the cycle. It will have a lag effect on the next bit and then the next bit the next bit. Adding plants to the equation can even have a knock-on effect on igneous geology [new rocks that come from volcanoes]. There is a study published recently that showed that the composition of some magmas over time has changed a little bit. And the reason it's changed a little bit is because the composition of the crust that is subducted has changed over time. One of the factors they cite in the study is this muddying of the continents by land plants.

STH: What's next for those studying the impact of land plants on the world?

WJM: I think that's a really cool question. I’ll keep my answer topical. NASA have just managed to successfully land their Perseverance rover on Mars. The reason we're going there is because we want to find mudstone. Mudstone is a highly sought out astrobiological target because it has this capacity to preserve organic matter.

The rover will sample the mudstone, and bring it back in 2031 — and some of it might be coming to Cambridge. They will look at the clay minerals inside these muds. Some of these minerals are produced biotically and some of them are produced abiotically. Now what we started to do in our line of work is look at pre-Cambrian muds — pre-vegetation muds — and actually look at the minerals that make it up. They are all this type that form abiotically. So they've got a different petrological signature to the biological equivalent. All you have to do is find one type of clay mineral on Mars that you know from an Earth analogue can only form biotically. Then you can do backflips! Then you have found your evidence for life. Convincing the general public, that'll be another thing!

How Rivers Shape the Planet We See Today

At a global scale, rivers act as nutrient highways — providing the oceans with manna that sustains life in the deep blue. The collision of tectonic plates, over millions of years, heaves rocky material to the surface forming mountain belts, which are subsequently pummelled by monsoon rainfall over similar timescales; a process that removes CO₂ from the atmosphere. Eventually, the rocky titans are diminished to no more than rolling hills. The products of these weathering reactions are transported by rivers to the oceans, where plankton and other organisms such as oysters, mussels and corals, utilise the nutrients to construct their shells. Without mountains and structured, efficient, meandering rivers, shelled seafood would be very scarce; something to ponder over during your next moules marinière. The huge numbers of plankton that have benefitted from the nutrients supplied by weathering will eventually die; their shells will sink and some will find their way to elevated portions of the seabed and form limestone and chalk beds, such as those in Dover, England. Those unlucky plankton that do not come to rest on an oceanic ridge will be dissolved in the caustic bottom waters of the ocean, the dissolved products will one day, after a few thousand years, reach the ocean surface via upwelling and release a tiny amount of CO₂ into the atmosphere. Through ocean ridge spreading and tectonic collision, the lucky plankton may be uplifted or subducted. If uplifted, mountains are formed and the process begins again; if subducted, the plankton will be melted and outgassed as CO₂ into the atmosphere through volcanoes. To observe the part of this cycle that regulates global climate — known as weathering — scientists use rivers as their periscope.

By studying the chemistry of some of the world’s largest rivers, scientists are edging closer to understanding the large role weathering plays in regulating atmospheric CO₂ concentrations. Canonically, climate is regulated on long time scales in two ways; the weathering of silicate rocks (like granite) with slightly acidic rain (this dissolves atmospheric CO₂), and the exportation of organic carbon from the continents to anoxic (oxygen absent) portions of the oceans. These two processes sequester carbon in the deep ocean for millions of years; it is thought that silicate weathering and organic carbon export have regulated temperature and kept Earth habitable for ca. 3.5 billion years. Understanding the chemistry and particle physics of rivers gives scientists a chance of quantifying the climate regulation ‘work’ that rivers do in the modern day. To understand this further we interviewed Dr Ed Tipper of the University of Cambridge. Dr Tipper is a geochemist studying ‘flowing carbon’ in Himalayan mega-rivers such as the Irrawaddy, Salween, and Mekong to name a few. Dr Tipper and his research group undertake yearly field campaigns to try and understand how these chemical reactions are regulating climate on million-year timescales.

So why the Himalayas? ‘The Himalayas are one of the most rapidly eroding areas on the planet, they are so-called “weathering limited” because the supply of material to the weathering reactor is very high, as a result, weathering fluxes are amongst the highest in the world’. By weathering reactor here, Dr Tipper is referring to the thin veneer of sediment (up to a metre below the surface) that is actively being chemically weathered. The term weathering limited refers to the fact that the supply of material is greater than removal during monsoons! The system is therefore always in excess of weatherable material, this makes the Himalayas a great place to study climate regulation via the mechanisms discussed previously. Dr Tipper goes on to talk about the specifics of his Himalayan fieldwork; ‘The goal of the work is to understand the weathering processes and quantify weathering fluxes (using rivers). It’s an interesting idea as the transport time of river particles is longer than the residence time of the water (residence time refers to the amount of time water spends within the river system), so there is an opportunity to determine time-integrated weathering rates. Another major goal of our Himalayan work is to consider the role of tectonics in mediating the supply of matter to Himalayan rivers. This work began after the 2015 Gorka earthquake and aftershock’. Through studying the chemistry and sediment characteristics of rivers in Himalayan rivers, Dr Tipper et al. are able to understand how weathering, hence climate regulation, varies with both time and material supply (from tectonic interventions like earthquakes).

Dr Tipper’s most recent research utilises a global river dataset to show that silicate weathering probably plays a lesser role in climate regulation than first thought. This means some other mechanism, such as organic carbon transport, is doing far more climate regulation work than canonically understood. Dr Tipper explains: ‘Large rivers transport water and sediment to the floodplains and oceans, supplying the nutrients that sustain life. They also transport carbon, removed from the atmosphere during mineral dissolution reactions, which are thought to provide a key negative climate feedback on long time-scales. We demonstrate that the (million year) carbon flux associated with mineral dissolution has been over-estimated by up to 28% because of a reactive pool of elements transported with river-borne suspended sediment. This is most acute in regions of high erosion, where silicate weathering is thought to be most intense’. The piece was published in PNAS in January 2021.

On a global scale, the role of the river is obvious. As arteries deliver nutrients to organs, rivers deliver nutrients to oceans and in doing so sequester carbon from the atmosphere, storing it on the seabed for millions of years. Observations of river chemistry and physics have put our current anthropogenic climate change in the context of natural slow processes that have regulated climate for billions of years. Current CO₂ emissions outpace any natural CO₂ removal mechanism and geo-engineering strategies must be implemented to achieve any reduction in warming during the next century. By understanding and observing the processes that regulate Earth’s climate in rivers, scientists can develop new modelling approaches to predict how our anthropogenic perturbation may impact global element cycling in rivers for years to come.

Modern Mega-Rivers in Rural Communities

Intricately coupled to rivers and anthropogenic climate change are the people and communities who make a living out of rivers. The Mekong Delta, referred to as the ‘Rice Bowl’, is the most prominent irrigated rice system in Vietnam, producing on average 20 million tonnes of rice per year. Without the constant supply of nutrients and sediment from the Mekong river this invaluable resource would diminish to nothing. In order to feed the world without costing the Earth it is critical to protect places like the Mekong Delta, especially as 50% of the world’s population rely on rice as a staple food. However, the humble rice grain is not the sole interest of those concerned with the Mekong, where separate industries include sand mining and damming for hydro-electric power (HEP).

Vital for construction, sand is the main constituent in mortar, concrete and some varieties of bricks. The inconspicuous sand grain is the second most extracted resource on Earth, behind water. Strolling through the streets of Cambridge you may be unknowingly treading on sand dredged from the Mekong, which once was probably part of the Himalayas, one way to feel on top of the world. In a recent Nature Sustainability paper, a set of novel acoustic experiments are used to determine the sediment flux entering the Mekong delta. Alarmingly, the study concludes that only 6.18 ± 2.01 Mt/yr (1 Mt = 10⁶ tonnes) of sediment enters the Mekong Delta, whilst current sand extraction rates are 50 Mt/yr. This mismatch in supply and demand has detrimental impacts for the Mekong, and potentially other large rivers subject to sand mining. A separate issue with similar consequences is damming. The rush to industrialise in a carbon conscious world has resulted in the construction of eleven major dams on the Mekong, with more planned for the future. The dams provide much of South-East Asia with electricity, but in some areas have reduced the once bloated silt rich river to a shadow of its former self. A paper published in Science of The Total Environment uses a sediment model coupled to future climate projections to predict the impacts of damming on sediment fluxes in the Mekong. In the next two decades, the model predicts the suspended sediment flux in the lower Mekong will decrease by 50% compared to current levels; that is despite an increase in monsoonal intensity as a consequence of climate change.

Dr Chris Hackney, a geomorphologist and lecturer at the University of Newcastle, was one of the scientists involved in producing the research previously mentioned. Along with colleagues from the Universities of Southampton, Hull, Oxford, Exeter, Illinois, Montpellier, and New York, this research group have been studying the dynamics of sediment transport in the Mekong river and delta for close to a decade. We interviewed Dr Hackney to gain a deeper insight into the multifaceted implications of sand mining and damming on the Mekong and its delta.

What are the main physical impacts of sand mining and damming on the Mekong? ‘Sand mining and damming remove sediment from the river basin. The Mekong delta has lost ~70% of its sediment load in the last few decades as it’s being trapped behind hydropower dams upstream — what sediment reaches the delta is removed for construction by mining’. Dr Hackney goes on to explain the implications of this sand removal on the Mekong. ‘Sand removal has resulted in the gradual lowering of the river bed throughout the delta and upstream in Cambodia, this means saline seawater is propagating upstream into the delta, affecting rice crops and agriculture. Work I led, published in Nature Sustainability last year, has shown that a three metre drop in river bed level is enough to shift river banks to an unstable condition — current rates of bed lowering are 10-20 cm a year. It’s conceivable that in a decade river banks may begin to fail at an increasing rate’.

The impacts of sand removal are detrimental to the physical processes that make the Mekong what it is, but what about the people who live there? Do the benefits of HEP and sand mining outweigh the negatives? ‘It’s important to remember that HEP and sand mining have positive and negative aspects. As the Mekong countries develop and their economies grow, the need for energy and material for construction does too. Both HEP and sand mining act as income for the residents in the Mekong basin — improving livelihoods for millions of people. However, as mentioned before, the environmental impacts being felt will ultimately impact livelihoods in a negative way. Negative impacts include the loss of land and infrastructure due to coastal/bank erosion, changes in salinity reducing agricultural yields — meaning people may be unable to provide for their families, and finally HEP and sand mining have a major impact on fisheries, which millions of people rely on for food’.

What’s the solution — are policy interventions in the process of being made? Is there a ‘one-size-fits-all’ solution? ‘This is a really important question, at the moment I don’t think there is an answer. Society needs sand to function — a demand that will not go away overnight. Alternative building and construction materials may be found in the long run, but right now ensuring the sustainability of the sediment supply through the Mekong is vital to maintain ecosystems and biodiversity of the region. This may involve limiting mining to areas where there are large sand deposits, or limiting mining to periods of the year when sand supply is high. What is clear, however, is that greater regulation and environmental impact assessment is needed across the Mekong, and other river systems to ensure we do not adversely impact on the natural environment’.

As a point of interest — how do you actually measure a sediment flux in such a large, deep, fast flowing river? ‘In my research, I use high-resolution acoustic techniques. Acoustic Doppler current profilers work by sending out pulses of sound through the water column. These sound waves reflect off particles in suspension and back to the sensor on the boat — telling us how fast these particles (and the water) are moving. The strength of the signal also tells us something about the amount of sediment at a particular depth. Similarly we use acoustic techniques to map the river’s bedload (larger particles transported along the river bed) at centimetre resolution, using a multi-beam echo sounder. This is effectively sonar — by repeating surveys of the same parts of a river over hours or days, we can track bedforms (such as dunes) moving along the river bed’.

By observing the Mekong we see how intricately linked society is to modern day mega rivers. By utilising acoustic techniques we are capable of monitoring the health of these rivers, and observe the huge impacts humans can have on our vast life-lines that endeavour to keep the planet in good health.

Rivers provide the observer with a spyglass into the inner workings of Earth’s climate system. Since their inception over 400 million years ago rivers have carried mountains and carbon to the oceans, and in doing so regulated Earth’s climate and sustained ocean life. Observing river morphology and chemistry not only provides scientists with information about how we may engineer climate sustainably, but also imparts real-time information concerning the health of our Earth’s arteries. Depended on by millions of people, current unsustainable practices, such as sand mining, require immediate policy interventions to facilitate food security and livelihoods in both the developing and developed world.

Will and Séan are both second year PhD students at Magdalene College studying Earth Science. Artwork by Mariadaria Ianni-Ravn.