Sedimentary Crisis at the Global Scale 1: Large Rivers, From Abundance to Scarcity

By Jean-Paul Bravard

The Earth’s oceans are currently undergoing unprecedented changes: rivers have suffered a severe reduction in their sediment transport, and as a result, sediment input to the oceans has dropped lower than ever before. These inputs have varied over millennia as a result of both natural occurrences and human actions, such as the building of dams and the extraction of materials from riverbeds.

Sedimentary Crisis at the Global Scale 1 examines how river basins have been affected by the sedimentary crises of various historical epochs. By studying global balances, it provides insights into the profound disruption of the solid transport of fluvial bodies. The book also explores studies of various rivers, from the Amazon, which remains relatively unaffected, to dying rivers such as the Colorado and the Nile.

• Language: English
• Publisher: Wiley
• Release date: Mar 7, 2019
• ISBN: 9781119579854

Book preview

Foreword

Rivers and deltas have been essential to the human enterprise on planet Earth since the dawn of civilization, along the Tigris and the Euphrates, the Nile, the Yangtze and the Indus. Rivers provide domestic, commercial and industrial users with water, not to mention irrigation for agricultural purposes; they are a means of transport, a habitat for fauna, a source of hydroelectric production and fertile river sediment to enrich the floodplains. Some of them carry solid sediment in the form of tailings, which may affect aquatic ecosystems and be deposited on the floodplains. Transported sediment comes from the bed and banks of the river as well as the watershed of the highlands, which has consequences for the stability of the channel, every use of rivers and its floodplains. Rivers transport more sediment to the world’s oceans than all other agents, such as glaciers, groundwaters, wind, volcanoes and waves. When water rich in energy and sediment enters a low-energy water plane, low-altitude deltas are formed. These are as important for humans as they are for fauna. They can serve as a buffer against floods, but less and less efficiently as they sink and are occupied, over time, by humans.

Rivers and deltas have been struck by natural disasters like hurricanes and floods. They have also been transformed by humans over the centuries, through direct manipulation in the form of dams, levees, diversions, bank stabilization, channeling/recovery, and the extraction of sand and gravel. Rivers and deltas are also indirectly affected in the watershed by human activities that modify the delicate balance between the water and sediment in the river system and that bring about change in the vegetation (deforestation/reforestation, grazing livestock, forest fires and agriculture).

One of the greatest challenges facing scientists who study rivers and deltas lies in distinguishing the impacts of human activity from the changes that would have taken place without human interference. This challenge demands an integrated understanding of the hydrologic cycle, fluid dynamics, sediment cascades, water resource engineering, the history of the use of land and water, water quality, and the transport of contaminants. Professor Bravard takes inspiration from all of these subjects, which contribute to a better understanding of the role of the river and the delta over time. This book begins with a study of the changes in European rivers, which began in the Middle Ages and continued throughout the Little Ice Age. In the late 19th Century, the impoverishment of river sediment became a major problem, with unexpected consequences for mankind. The reader will be impressed to discover to what extent rivers and deltas have changed over time, to what extent humans and natural factors have exercised their influence and to discover the consequences of these changes on human activities.

It is difficult to imagine an author more qualified than Professor Jean-Paul Bravard to write a book detailing the geography and history of the rolling carpet of sediment from river sources, the transport through their river gutters and deposition at their deltas. In this book, Bravard compiles the lessons gained from his education and his own experience to discuss the change in character and behavior in rivers and deltas. Bravard describes and explains the different forms that this metamorphosis takes through the use of striking examples in Europe, Russia, China, North America, the Amazon, Egypt and Asia.

Bravard has compiled a research file that has earned him international recognition for his work on rivers and deltas, detailed in multiple books and more than 350 articles in highly renowned national and international journals. This led to him being recruited as a consultant for the Rhône and Loire authorities and, more recently, the World Wildlife Fund for Nature in Southeast Asia. He has also received the CNRS1 Silver Medal in Human Sciences for his work. In this volume, his description, explanations and predictions concerning rivers and deltas are now synthesized for the informed public.

Richard A. MARSTON

University Distinguished Professor at Kansas State University

Editor-in-Chief of Geomorphology, 1999–2018

Former President (2005–2006) of the American Association of Geographers

Member of the AAAS, AAG, GSA, Explorer’s Club, Royal Geographical Society

1 National Center for Scientific Research (Centre national de la recherche scientifique).

Preface

For the past 20 years, scientists, NGOs and a portion of public opinion has been concerned with the threats weighing on the globe’s deltas. The most visible way in which this danger manifests itself is through the multiplication of disasters linked to cyclones; they cause considerable casualties and extensive damage to a number of coastal communities. In fact, coasts around the world seem to be more severely affected than they used to be by the violence of winds and storms resulting from the rise in the sea level. The issue of climate change must not, however, turn our attention from other crisis factors experienced by deltas. These actually accumulate major weaknesses that are important to their very nature: their flatness, their very limited altitude connected to their young age, a withdrawal from the coast and the increasing severity of natural recess. Deltas, which demonstrate fertile land and are highly populated, are, in many cases, also lands of suffering, because, in addition to their exposure to natural phenomena, they also suffer from the over-exploitation of their resources, a lack of water that seems paradoxical and, sometimes, war.

However, deltas are not autonomous spaces. Owing to the very fact that they are at the river mouths of continental surfaces that have been occupied for millennia by dense human societies, they are also subject to dynamics much more directly connected to natural processes and human actions of a continental nature. Deltas actually owe their existence to the deposition of fine sediment carried by rivers to their outlets into the sea. These contributions have varied over the centuries and millennia as a response to changes in the climate and the effect of human actions on erosion in basins; river basins around the globe have experienced periods of intense sedimentary transport contrasting with periods of limited transport. Deltas are subject to the acceleration in the rise of the seas and climatic hazards, but to a large extent, they can respond and adjust on condition that the contribution of sediment with a continental origin maintains sufficient intensity. In the opposite case, these contributions reduce in intensity and deltas are or will no longer be able to compensate for their recess or the rise in the sea level, and this is the case for most deltas around the world today.

Why are the links between continental spaces and deltas considered distended or cut off? If the need for sedimentary contribution to deltas has been proven today, why were these contributions unceremoniously intercepted in the 20th Century? Could the effects of the rising sea level have been misevaluated or neglected, leading to the burning issue today? Or is it rather because historic continuity, which leads sediment from hills and mountains to the sea, was not understood, or was forgotten, or even considered negligible, inconvenient for certain practices in place in river systems? In short, what are the root causes of the contemporary crisis of deltas? These are the questions this book will attempt to answer. Its goal is to raise awareness, beyond the scientific circle, of this complex phenomenon, because it lies at the intersection of natural processes and human choices. The situation is serious and implies understanding the nature of the exchanges between rivers and oceans around the globe, particularly at deltas, which are also affected by the effects of climate change.

To respond to the questions from the preceding paragraph, we have made the methodological choice to first understand and present the history of rivers in order to better target the present and future of deltas, because they owe their existence to rivers. We will start by explaining where the contemporary landscape of Europe’s rivers comes from; then, we will ask what Europe has learned and passed on to the heirs of its remarkable intellectuals. We will note that the knowledge regarding the theoretical operations of rivers has not brought us to ethical practices, concerning both rivers and their natural extension, deltas. The chapters of this work are introduced below. Their order tends to suggest that the best river science does not necessarily bring about better practices, and that bad practices have a cost that society must always pay in one way or another.

The delta, less frequently flooded, in channels less traveled for transportation, is abandoned to its fate when stability seemed certain in the long term. The progress of technology and practices seemed to have made upstream–downstream solidarity useless in the mid-20th Century. However, for the first time in human history, the deltas have brutally become the victims of the improper management of continental waters, for the sedimentary balance* is off. Hydroelectric development has been the major cause of the fragmentation of the elements of the hydrographic network and drainage basins, at the same time that the extraction of river resources was experiencing a considerable, uncontrolled rise, including in deltas themselves. It is rare to observe the convergence of bad policies, even though the responsibility for the current disorder weighs, first and foremost, on the management of continental river systems.

Acknowledgements

The author would like to say a big thank you to Yves Bégin, Geneviève Bravard, Marc Goichot, Richard Marston, Michel Meybeck and Laurent Touchart for their shrewd advice in the editing phase; Thierry Sanjuan for his original suggestions; Colette Bedoin for her presence during a difficult technical stage and Emmanuelle Szychowiak for her translation of a German text.

He would also like to express his gratitude to Richard Cosgrove (North Canterbury Fish & Game), Neil Cullen (New Zealand Farm Forestry Association), Marc Goichot (cover of volume 1), Robin Gruel, Allan James, Lois Koehnken, Ingrid E. Luffman, Georges Pichard, Peter Scott and Vivian Stockman (OVEC) for granting reproduction rights.

The resources provided by Gallica were also extensively used in this work.

Jean-Paul BRAVARD

December 2018

Introduction

The first volume of this work starts with the history of a crisis that affected Europe in the late Middle Ages and the modern era. This crisis extended from the mid-14th Century to the mid-19th Century, five centuries dominated by misfortune, from the mountains to the mouths of the valleys. The mountains actually saw great torrential activity, whereas the rivers saw frequent and serious rises and floods; the damage they caused in a context of generalized cold seasons was considerable. However, crisis periods have luckily been broken up by dryer and warmer periods that have allowed the populations to grow again. We will seek evidence of the crisis from the Italian Apennines to Northern Europe via the Western Alps. There is no doubt that this crisis was under the primary influence of a damaged climate. This is attributed to the hydrological effects of the Little Ice Age* that affected harvests, brought about famines, caused high morbidity* and maybe even caused conflicts on a continent that otherwise saw a remarkable economic and intellectual boom. Climate is not everything, however, for the mountains were spaces of colonization and intense valorization over the centuries when the low agricultural productivity and the need for variety were the norm.

The hydrological crisis of the Little Ice Age in the northern hemisphere stimulated both intellectual research and the boom in empirical knowledge. Chapter 2 deals with the slow conquest of hydraulic knowledge by the European powers, France, Italy and the Netherlands. The choice to use these countries as examples is partly guided by the fact that they have rivers that drain water and sediment from fragile mountains towards their deltas. Some basins are full-scale laboratories for the development of the erosive crisis that they experienced between the late Middle Ages and the 19th Century; they lend themselves remarkably to a cross-analysis of scientific progress. Italian engineers, with their mathematical training, were prematurely at the cutting edge in this domain in Europe because prosperous cities and their contado* directly confronted the risk and were to efficiently reduce their destructive effects to guarantee urban prosperity; France, on the other hand, had more theoretical perspectives, primarily focused on calculations, most likely because wars and the need to make commerce prosper demanded other approaches, notably the construction of a network of canals1. We hope to show that European hydraulic science is constituted in the modern era and that it saw its apogee in the mid-19th Century. The notions of drainage basins* and upstream–downstream solidarity are derived therefrom and are, as we will observe, result including all the logic of the great erosive crisis from the Little Ice Age. A detailed understanding of the processes at work in drainage basins led to the deletion of some of the causes of the crisis, at least those that stemmed from human action. Thus, the mountains were protected from the effects of erosion, to the benefit of river mouths.

In a context marked by instability, we feel that the excesses of the sky and waterways were feared by the affected societies. Mastery of the waters and land in motion was the obsession of societies in a state of crisis, an obsession that, incidentally, lasted well beyond the crisis itself in the mountain valleys. They are therefore places where the simple rise in the water level revives old fears and calls for arrangements that are the direct heritage of old practices. Mountains up to the sea, engineers and suffering populations have mobilized themselves to find means to escape misfortune. However, the river crisis, that of the risk of flood and that of damaged lands, of production, and of poverty, has also been a harbinger of collective progress. The solidarity between territories led to the appearance of the notion of the drainage basin, as well as the principle of the spatial uniqueness of water management. Never have industrializing agricultural societies better designed river management than in the 19th Century, fully aware of the connections between the mountains and the sea, like the interactions between slopes, rivers and deltas.

Since the Neolithic agricultural revolution, which started in the Middle East and affected most of the regions of the globe, the continental balances have been affected and, in several large basins, the river imbalances were nearly contemporary to the construction of deltas that are hardly 6,000 years old. One powerful destabilization factor is the factor of agriculture: it involves clearing and scraping soil prepared for agriculture. Some specialists suggest that agriculture brought the Earth into a new geological period, the Anthropocene, characterized by formations of the Earth’s surface and geological formations produced by human action. This is a broad subject that does not lie in the direct perspectives of this work, but these goals largely overlap. Archeology and geoarcheology teach us a great deal about the chronology of the episodes of humans taking and losing influence at the surface of the land that has emerged, on the disappearance of protective forests, on the sedimentary flows that have overcome the oceans, built the floodplains, left proof of human action in lakes and, thus, allowed this action to be quantified. Societies have been powerful destabilizing agents, to the extent that they profoundly changed the sediment balance around the globe and gained the upper hand over natural destabilization factors, at least during certain periods. The effects of several and long-lasting climatic crises superimposed themselves on a background of growing agricultural impacts – growth certainly unequaled in space and time – in certain periods of the humanized Holocene*. Periods of cold and heavy precipitation hit certain regions of the globe hard, notably in the northern hemisphere. Crises concerning production and even morbidity were accompanied by the loss of fertile land, materials being swept away by rivers that then rose while the adjacent plains were covered with marshes, the river dug into the earth and the deltas progressed. It is important to make a distinction between mountain erosion (concerning rocks and surface formations*) and the erosion of soil stricto sensu, which supports agriculture (even though the soil was the first to be eroded on the slopes of the Alps). The phenomenon of soil erosion is ubiquitous and we will only deal with a few examples of accelerated erosion.

The change was brutal starting in the late 19th Century, a period in which a process of great sedimentary drying up concerning most rivers around the globe took place. To present this, it is necessary to understand, before and in light of the contemporary discoveries, the chain of processes that follow one another from the mountains to the sea. The numbers obtained through the extrapolation of values measured on small eroded surfaces in river basins are quite different from the values measured at the mouths of rivers. This is because drainage basins contain countless natural traps in which sediment transported by the waterways that drain them are deposited. The numbers confirm that agricultural development (as well as land abandonment) was indeed the major factor disturbing natural sedimentary balances, whether or not the climate played a magnifying role on weakened lands. New, powerful factors have appeared in the past 150 years, though. Around the world, the extraction of useful mineral resources is taking place in the very beds of rivers. It crosses channels, reduces the risk of floods and allows floodplains to be developed by agriculture, cities and industrial zones. However, the primary factor is the construction of large reservoirs that modify hydrology to the detriment of transportation and trap considerable quantities of sediment. The dam-reservoir has segmented the river space and led to the solidarities between upstream and downstream being forgotten. In Europe, the late 19th Century was a period of demographic decline in the mountains, spontaneous and planned reforestation, and the progressive mastery of erosion. With mountains whose erosion has largely been mastered through sections of rivers equipped with reservoirs, the river has lost the logic of upstream–downstream solidarity.

There are several models of river disturbance that share the general processes presented in this introduction, but they decline these specifically in the form of original geographic complexes influenced by the geology and topography of their drainage basin, by their climate and by the relative weight of the human pressures imposed upon them. We have chosen the Amazon as an example of a relatively undisturbed large river, even though the threat of large dams is becoming a reality. The situation of large rivers in China and Southeast Asia, where development is taking place at a rapid pace and producing brutal impacts, is another matter altogether.

We have just left a century that saw the greatest breakthrough in the Holocene period. In the last millennia, which were those of the humanization of the Earth’s surface, reversibility was still present: instances of deciphering and climatic crises could destabilize the environment of river basins, but the crisis phases were followed by more or less long periods in which the previous balances were restored (or at least new balances, close or far from the previous, were put in place, the original state of reference not existing). Deltas have recorded the crises and remissions and progressed, taking each year as it came; there has been nothing of the sort in the last decades in which we have brought out a new world, that of delta penury. This first volume provides the foundation of the second, which will be dedicated to deltas.

1 Hydraulic science was not born in Western Europe, but in China and the Middle East; it is one of the facets of the Renaissance in Europe.

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1

The Torrential Crisis in the European Mountains (14th–19th Centuries)

1.1. Introductory generalities on global fluvial systems

Fluvial systems function according to universal principles, governed by fluid mechanics, while remaining subject to regionalized constraints under climatic control; this provides partially distinct forms to northern, temperate and tropical rivers. In most matters and to stick to the circulation of water and sediment, the scientific literature distinguishes sediment production zones* (essentially localized in mountain and hill regions), sediment transfer zones* and sedimentary deposit zones downstream (alluvial plains, deltas, ocean margins). This longitudinal zonation is also governed all over the globe by secondary processes that nuance the general principle.

First, river styles* are under the control of climate, vegetation that more or less protects slopes, and geology that conditions lithology and the ability to mobilize certain types of soft materials. If the balance or equilibrium between liquid flow (capable of ensuring sediment transport) and solid flow (sediment to be transported) is in favor of the former, the material is easily evacuated and the river presents a simple morphology, demonstrating a single winding channel, sometimes classified as a meandering* channel. These channels are found in regions that produce little sediment (mountains and hills with heavy rainfall, wooded, sparsely populated) and in the transfer zone.

If, on the contrary, the liquid/solid fluxes balance is in favor of the latter, then the flows are not capable of transporting the entire load originating from the production zone. This situation is very frequently seen in regions with semi-arid climates (the slopes there are badly protected), in regions of the globe with a contrasted relief and a fragile structure, and finally in elevated areas that have been cleared and over-exploited for pastures and agriculture (Figure 1.1).

Figure 1.1. The Selle torrent in the Massif des Écrins (France); it is fed here by an avalanche cone whose base is eroded by lateral erosion from the torrent. The bed comes loose downstream and adopts a braid style

(source: J.-P. Bravard)

The production zone and sometimes even the transit zone are then saturated with rough material; in response, the river channel adopts a particular shape and mode of functioning, braiding*.

The braid style is classically contrasted with the meander style, even though composite or hybrid styles are frequently seen. The excess sediment therefore has a descriptor, or marker, which is braiding, easy to diagnose and interpret based on the dynamic function of the basin. Without anticipating too much further, we can assume that braided channels reveal crises, which are the subject of the first chapters of this work.

Figure 1.2. A river in British Colombia, the Squamish River. This wandering gravelbed river*, with a basin well provided in both water and sediment, meanders and braids at the same time in a former glacial basin [HIC 75]

(source: J.-P. Bravard)

Some further questions have join those stated above, at the forefront of scientific investment. These concern:

1) the precise methods of mobilizing material on slopes until they enter channels (this is slope–river bed coupling* or decoupling);

2) revealing permanent deposit sites at the foot of eroded slopes far from active channels; these are long-lasting, for they are sheltered from erosion by channels (we generally speak of colluvia* to distinguish them from alluvia);

3) the importance of alluvial plains as deposit sites for alluvia brought when channels flood; the question also needs to be asked of these materials being recovered when the river channels move on the plain and recover these deposits to incorporate them into its load;

4) the chronology of sedimentary recovery that can ensure a transfer to the system’s traps downstream, even during phases with limited material entry into rivers from production zones. In short, the process chain involving production–transfer–deposit and the morphological continuum* involving mountain–piedmont plain–valley are not sufficient to correctly grasp the load of a river that is building its delta.

Finally, let us observe that the globe’s rivers, aside from increasingly rare exceptions, have stopped presenting the pure mechanisms, that were briefly described, for more than a century (sometimes longer). Sedimentary traps, the complexity of which was just explained, have moved to the background, behind the reality of artificial traps constituted by dams-reservoirs, the number and capacity of which exploded across the globe in the 20th Century.

However, let us move to the heart of the subject of this first chapter, which is the sedimentary crisis of the Little Ice Age (abbreviated as LIA). The knowledge we have of the LIA is progressing in the fields of both hydrology and sediment transfer and deposit, not to exclude the mechanisms that connect these two components. Let us take the example of hydrology. A very in-depth study of the archives of cities on the Lower Rhone has provided great insight into the hydrological rhythm of the river at the outlet of its basin [PIC 14b]. The study of the floods on the Rhône, spread across decades and four degrees of gravity based on their manifestation in the riverbed and in its floodplain, revealed new elements, particularly their classification into two hydrological hyperphases, the first dating back to 1450–1599 and the second to 1647–1711. The hyperphases are separated from one another by a period of moderate hydrology (1600–1646). On an even finer level, the decade 1701–1710 was the hardest in the history of the river, and before it, the period from 1481 to 1500. During hyperphase 1, an isolated event, the 1548 flood, would surpass even that of 1856, despite it being considered the most significant flood in the Rhône’s history, i.e. the absolute reference for risk managers in France. Another discovery is the way in which the contemporary hydrological regime was established, with boosts in strong hydraulicity in the decades 1770–1780, 1801–1810 and 1841–1850. If the floods in 1840 and 1856 do not belong to these sequences, it is because these are isolated manifestations on a foundation of weak hydraulicity (Figure 1.3). This figure is one of the markers of the river’s new hydrology according to the authors of the study, G. Pichard and E. Roucaute.

It is possible to evoke the reality and the gravity of the long crisis of the Little Ice Age (LIA), because the proofs of highly active processes that affected the torrents and torrential rivers are numerous enough to be convincing. These manifestations allow us to understand the extent of the means used by the inhabitants of slopes and floodable plains to find initially local and provisional solutions, then more or less definitive ones, occasionally benefitting from the help of the public authorities. We will consider the question of the torrential crisis from the LIA from the perspective of several European mountain ranges.

Figure 1.3. Hydrological hyperphases (black curve) and flood periods (red curve) from the 14th to the 20th Century. The gray line emphasizes the LIA period (source: [PIC 14b]). For full color image see: www.iste.co.uk/bravard/sedimentary1.zip

1.2. Manifestations of the LIA crisis in the river valleys of Western Europe

1.2.1. Mountain crises

Historians have collected testimonies in the