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A World of Rivers
Environmental Change on Ten of the World's Great Rivers
By ELLEN WOHL
The University of Chicago Press
Copyright © 2011 The University of Chicago
All right reserved.
ISBN: 978-0-226-90478-8
Chapter One
A Round River
Contemplating the lace-like fabric of streams outspread over the mountains, we are reminded that everything is flowing. JOHN MUIR
Along the networks of the world's rivers lives a disproportionate richness of plants and animals, all adapted to the rivers' yearly pulsing and occasional outbursts. Change either of the vital components-the supply of water or sediment-by altering the climate, by cutting the forests that stabilize the soil, or by damming the river, and the river itself changes in a cascading effect that influences all the rich life linked to the river corridor. We humans have been learning this fundamental yet highly complicated lesson over and over for millennia.
People too often view constantly changing rivers as inconveniences. We try to stabilize them by confining them in single straight channels that do not spill across the floodplain or migrate from side to side across the valley bottom. This confinement diminishes the complexity and diversity of habitat that nourish abundant and varied species of plants and animals. Where rivers regularly overtop their banks, floodwaters carry sediment and nutrients that rejuvenate the wetlands and forests of the floodplain. Fish disperse from the main channel across the newly flooded lands, where the warmer, shallower, slower-flowing waters that nourish the growth of microbes, algae, and insects also provide food and nursery habitat for vulnerable young fish. Floodwaters receding to the main channel carry organic matter that helps supply the river's food web. Ecologists identify this "flood pulse" as one of the primary contributors to river health.
In addition to spreading water, sediments, and nutrients across the valley bottom, floods can also reconfigure the river channel. Their greater energy enhances bank erosion, shifting channels across the floodplain and leaving depressions that become ponds or lakes, as well as secondary channels that are fully connected to the main channel only during floods. The presence of swift, deep flow in the main channel, slower flow in the partially connected secondary channels, and still water in the floodplain depressions creates a variety of habitats. Animals that live primarily in upland environments also spend time in these riverine corridors where resources are abundant. When dams reduce the annual flood pulse and channelization eliminates the diverse habitats of the river corridor, the complexity that supports abundant life is lost. Jürg Bloesch, president of the International Association for Danube Research, refers to uniformity as the illness of rivers, not only because of lost complexity and abundance, but also because of lost function. The filtering effect of riverside vegetation and floodplains is reduced or lost when the microbes inhabiting water and soil lose habitat. In consequence, the river is more likely to pass downstream any contaminants that it receives, from excess sediment and nutrients to pesticides and PCBs, rather than storing them or breaking them down into biologically less harmful compounds. Contaminants passed downstream reduce water quality along the length of the river and into the coastal environment. The physical buffering provided by rivers is also lost to increasing uniformity. Rather than spreading across broad floodplains and moving slowly downstream, floodwaters remain concentrated in the main channel and create destructively large and fast-moving floods that are more likely to damage structures and communities near the river.
This book explores how the changes humans impose can impoverish the rivers, and by extension impoverish all the many creatures that rely on them. To illustrate the nature of rivers, I have chosen ten of the world's largest. Among these the Amazon, Congo, and Mackenzie remain relatively unaffected by humans. The Ganges and Chang Jiang exemplify rivers undergoing rapid change as a result of increasing human alterations. The Ob-Irtysh, Nile, Danube, Mississippi, and Murray-Darling represent the variety of heavily altered rivers present in the world today.
These ten also exemplify the climatic, topographic, and biological variety present among the world's largest river basins. The tropical regions drained by the Amazon and the Congo pump enormous volumes of water and sediment into the adjacent oceans, whereas the Nile and the Murray-Darling emit only a comparative trickle from their arid drainages. The Amazon drops precipitously from the heights of the Andes, then meanders broadly for more than three thousand kilometers across an immense, nearly flat basin before reaching the Atlantic. The Ganges drops from the roof of the world in the Himalaya, then gradually turns and runs beyond the base of the mountains before making its way to the Bay of Bengal. The Congo traces a broad arc around its central depression before breaking through the Crystal Mountains along its lower course and tumbling down a series of cascades to the Atlantic. The Danube begins in gentle, hilly mountains, then alternates downstream between multiple channels as it flows across broad basins and through a single narrow gorge punched through the mountain ranges that cross its path to the Black Sea. The Amazon supports a tremendous diversity of fish species, whereas the Mississippi is particularly rich in species of mussels. Freshwater dolphins swim in the Amazon and the Ganges.
What the rivers share, besides their great size, is their crucial role in shaping the surrounding landscapes and biological communities. Rivers sculpt the lowlands they flow across by governing where sediment is removed through erosion and added through deposition. They also indirectly control the shape of adjacent uplands. Geologists working along the Indus River in Pakistan, for example, demonstrated that when the Indus cuts down rapidly, the neighboring valley sides become oversteep and unstable, triggering landslides that fill the valley bottom with sediment and temporarily slow the rate of downcutting.
Relative to the percentage of the landscape they occupy, river corridors host a disproportionately large number of plant and animal species. Rivers almost entirely dominate the transport of sediment to the oceans, with headwater regions producing most of the sediment. Biochemical processes along rivers and adjacent wetlands govern the amount of nitrogen, carbon, and other nutrients reaching coastal areas and thus strongly influence coastal and oceanic productivity. Rivers and wetlands also strongly influence groundwater storage by providing recharge zones where surface waters infiltrate to greater depths or by draining groundwater as channels cut downward and intersect the water table.
Despite the wealth of vital services and resources supplied by the world's rivers, human activities have impoverished most of the largest river basins. An assessment of fragmentation and flow regulation by dams published in 2005 indicates that of the ten large rivers profiled in this book, five (Mississippi, Nile, Danube, Chang Jiang, and Murray-Darling) are strongly affected catchments in which a substantial portion of the flow is regulated or the main channel and tributaries are highly fragmented by dams that effectively break the channel up into shorter segments. The remaining five rivers profiled here are moderately impacted catchments. Only 120 of the 292 catchments assessed for the study were judged to be unaffected as a result of fragmentation or flow regulation by dams, and most of these are smaller rivers.
Environmentalists sometimes quote John Muir, who wrote, "When we try to pick out anything by itself, we find it hitched to everything else in the Universe." Interconnections are particularly apt for describing rivers. Precipitation, windblown sediments, and atmospheric contaminants enter them from the air. Flying insects emerge from the river, and river water evaporates. Water carrying dissolved elements and compounds percolates down through the streambed to the groundwater, and groundwater seeps into river channels. Microscopic organisms and aquatic insects move back and forth between the river and the shallow subsurface, as do water and dissolved chemicals. Water, sediment, nutrients, and organisms flood across valley bottoms, then recede into river channels. Sediment and organic matter move from adjacent hill slopes and uplands into river corridors. And water, sediment, contaminants, and organisms moving downstream, as well as other organisms moving upstream, stitch together the uplands and oceans along the seams of rivers.
Aldo Leopold wrote of the functioning of an ecosystem as a "round river" to emphasize the cycling of nutrients and energy. This phrase could also aptly describe the planet, including the atmospheres, oceans, landmasses, and groundwater. I emphasize the importance of these global connections among seemingly individual river basins in the short "interludes" between river profiles. This book begins with precipitation over the headwaters of the Amazon. Following the river downstream across South America, chapter 2 ends where the Amazon enters the Atlantic. The intervening interlude traces the pathway of a hypothetical water droplet leaving the mouth of the Amazon until that droplet falls as precipitation on the headwaters of the Ob-Irtysh river system in Siberia. Each succeeding pair of chapters follows this format, tracing the path of the same water droplet down another river basin and then following it to the next one. In the interludes that trace pathways between river basins I have tried to estimate realistic rates of travel for a droplet moving with specific ocean currents and atmospheric circulation patterns. The travel times between river basins are underestimates, however, because many of the droplets moving down a river take much more circuitous pathways, with long periods of storage in groundwater, glaciers, or deep ocean currents. Polar waters take approximately a thousand years to circulate, for example, then hundreds of years more to surface at the equator, where they rise slowly from the ocean depths at only two to five meters a year. But water droplets do circulate around the entire globe, and contaminants, windblown sediment, and a wide variety of organisms, from microbes to whales, travel with them. It is this sometimes slow but always inexorable mixing that makes it ludicrous to think of any river basin in isolation. What people in the United States do in the Mississippi River basin does make a difference in the basins of the Congo, the Ob-Irtysh, and the Murray-Darling, and vice versa. We are all a part of one round river. The following chapters discuss some of the details of that river's ebb and flow using ten great rivers as pathways of learning and exploration.
My purpose in writing this book is to explore the natural history of some of the world's largest rivers and the history of human alterations to these river ecosystems by taking readers on a journey down each river and along the pathways that connect seemingly far distant rivers. My focus is on the science underlying our understanding of each river. Although I am fascinated by the approach of environmental historians, I do not explore the social history of people living in each river basin. The book is written to be accessible to nonspecialist readers interested in natural and environmental history, but the bibliographic sources for each chapter also provide reference material for those who wish to delve further into the topics discussed. Every river discussed here is the sole subject of several other books, as well as numerous scholarly articles. If the people of the world continue to impoverish the functioning of the greatest rivers, we cannot plead ignorance of the effects of our actions.
Chapter Two
The Amazon: Rivers of Blushing Dolphins
The Andes Mountains form a wall more than six thousand meters high in Ecuador, Peru, and Colombia. The mountain wall separates two of the most different regions on Earth by only eight hundred kilometers. To the west of the divide, the absence of water defines a landscape with jagged edges of rock and topography. Across the continental divide, the mountains drop abruptly into a vast green plain. Elevation drops from more than six thousand meters to approximately three hundred meters over a distance of three hundred kilometers. The plain is contoured by immense rivers that wind back and forth across it, and the abundance of water defines the landscape.
Water sifts down onto the high peaks as snowflakes that feed glaciers and icefields, and as snowmelt in the mountain streams. The Amazon River heads in a small spring that seeps from spongy grassland high on Mount Huagra. This spring grows into a small stream called the Huarco. As other streams fed by melting snow and ice join the river, it becomes the Rio Toto, then the Rio Santiago, followed by the Rio Apurimac. (Spanish-speakers use an accent on Río.) By this point the river has dropped down into cloud forest, moist and cool, but below the frost level. The name Apurimac comes from an Indian language in which it means "Great Speaker," for the steep Apurimac is filled with rapids and waterfalls. The downstream point where the rapids end marks the boundary between the crystalline rocks of the Andes and the younger sediments of the Amazon basin.
The noisy Apurimac becomes the Rio Ene, then the Rio Tambo, which flows into a main tributary of the Amazon, the Rio Ucayali. The Peruvians begin calling the river Amazonas at the confluence of the Ucayali and the Marañon. Amazonas remains the river's local designation until it enters Brazil, where it is known as the Rio Solimões until it reaches the confluence with the Rio Negro, when it is known once more as the Rio Amazonas. (Portuguese-speaking Brazilians neither accent nor capitalize rio.) Victorian naturalist Henry Walter Bates coined the name the Rivers Amazon in recognition of the different local names given to the great river. The plural seems particularly apt in this land of rivers.
By the time the flowing water reaches the Ucayali, rains are abundant. Torrential "male" rains cascade from thunderheads over the feathery stands of palm trees. Gentler "female" rains moisten the leaves of the enchanted broccoli forest where hundreds of tree species create an undulating canopy of varied shapes and sizes. Water seeps into the ground and collects in the stream channels, swelling them to rivers that overflow their banks and flood for tens of kilometers across the surrounding lowlands, creating vast lakes and flooded forests. The water's force cannot be contained, and the rivers continually shift within their channels, eroding banks here, depositing sandbars there, jumping abruptly to a new channel. Each river sweeps back and forth in extravagant loops, doubling on itself, trailing secondary channels, chute cutoffs, and oxbow lakes in its wake.
The city of Iquitos, Peru, rises abruptly within the dense forest and network of rivers, without a gradual buildup of rural areas and suburbs. Iquitos remains unconnected by the network of roads so ubiquitous elsewhere at the start of the twenty-first century. Two hundred thousand people live in Iquitos, and everything they do not make themselves comes in by airplane or by boat. During the flood season, water rises up the stilts of houses on the floodplain. Each year the river can rise and fall here by nine meters vertically. Many structures are built on floating rafts.
People have been living in Iquitos since at least the fifteenth century, but the settlement became a city with the rubber boom that brought both intense suffering and lavish prosperity to the Amazon basin early in the twentieth century. Rubber has not boomed in Iquitos in many decades, but other resources can be extracted. The city has large sawmills where the giant kapok trees (Ceiba pentandra) that rear like massive umbrellas above the surrounding canopy are peeled to make plywood. A more limited harvest now occurs following a decade of intense logging that created a scarcity of kapok trees, but recent oil discoveries suggest to some residents the promise of another resource extraction boom.
Downstream from Iquitos the river alternately narrows and then widens into a broad inland sea. Birds screech loudly along the riverbanks. The turbid water resembles café au lait. The river surface holds a complicated topography of wrinkles and calms, and upwellings where big boils rise off submerged dunes. The water is not uniformly opaque. Particles of silt and clay churn within it like split pea soup being stirred, and the sediment forms subtle and complex honeycomb patterns in shades of brown and golden brown. The depths remain inscrutable.
(Continues...)
Excerpted from A World of Rivers by ELLEN WOHL Copyright © 2011 by The University of Chicago. Excerpted by permission.
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