Rivers Research Paper

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The geographer Lewis Mumford’s observation— that all great historic cultures thrived by traveling along the natural highway of a great river—is particularly resonant today, as the world’s rivers bear the brunt of human manipulation. Pollution and habitat loss (two main side effects of hydraulic engineering), as well as climate change, pose unprecedented challenges to agriculture, manufacturing, urban water supplies, and wildlife conservation.

By common definition, a river refers to water flowing within the confines of a channel (as suggested by its Latin root, ripa, meaning “bank”). More precisely, a river forms the stem of a drainage system that transports water, soil, rocks, minerals, and nutrient-rich debris from higher to lower elevations. In a broader sense, rivers are part of the global water cycle: they collect precipitation (snow, sleet, hail, rain) and transport it back to lakes and oceans, where evaporation and cloud formation begin anew. Energized by gravity and sunlight, rivers sculpt the land around them, wearing down mountains, grinding rocks, and carving floodplains through the earth’s crust. As carriers of water and nutrients, rivers also provide complex biological niches for fish, sponges, insects, birds, trees, and many other organisms.

Rivers can be compared by basin size, discharge rate, and channel length, although there are no universally recognized statistics for any of these dimensions. The Amazon forms the world’s largest drainage basin at approximately 7 million square kilometers, followed distantly by the Congo (3.7 million square kilometers) and the Mississippi-Missouri (3.2 million square kilometers). As regards discharge rate, the Amazon once again stands supreme at around 180,000 cubic meters per second, followed by the Congo (41,000 cubic meters per second), the Ganges- Brahmaputra (38,000 cubic meters per second), and the Yangzi (Chang) (35,000 cubic meters per second). At around 6,650 kilometers, the Nile is the world’s longest river, followed closely by the Amazon (6,300 kilometers) and the Mississippi-Missouri (6,100 kilometers). Other rivers with large area-discharge-length combinations include the Ob’-Irtysh, Parana, Yenisey, Lena, Niger, Amur, Mackenzie, Volga, Zambezi, Indus, Tigris-Euphrates, Nelson, Huang He (Yellow River), Murray-Darling, and the Mekong. Two rivers—the Huang and Ganges-Brahmaputra—also stand out for their huge annual sediment load, which makes them especially prone to severe flooding. There is no agreed-upon minimum size, length, or volume for a river, but small rivers are usually called streams, brooks, creeks, or rivulets. Great or small, rivers that form part of a larger drainage system are known as tributaries, branches, or feeder streams.

Although rivers vary greatly in size, shape, and volume, most rivers share certain characteristics. A typical river has its headwaters in a mountainous or hilly region, where it is nurtured by glaciers, melting snow, lakes, springs, or rain. Near the headwaters, swift currents prevail and waterfalls are common, owing to the rapid drop in elevation or the narrowness of the valley though which the river flows. As a river leaves the high region, its velocity typically slackens and its channel begins to meander, bifurcate, or braid. Often its floodplain broadens as it picks up tributary waters. As the river reaches its mouth, it usually loses most of its gradient. It becomes sluggish, allowing some of its sediment load to settle to the bottom, clogging the channel. The river responds by fanning around the sediment deposits, classically forming the shape of a delta (the fourth letter in the Greek alphabet), before emptying in a lake, sea, or ocean.

Unique climatic and geographic conditions determine a river’s annual discharge regime (its seasonal variations in water quantity), but as a rule rain-fed tropical rivers flow more steadily year-round than do snow-fed temperate rivers. A river is called perennial if it carries water all or almost all of the time, and intermittent or ephemeral if it does not. A water-carved channel in an arid region is often called an arroyo or dry gulch if it carries water only on rare occasions. For hydrologists, the term flood refers to a river’s annual peak discharge period, whether it inundates the surrounding landscape or not. In common parlance, a flood is synonymous with the act of a river overflowing its banks. In 1887, a massive flood on the Huang caused the death of nearly a million Chinese people. In 1988, flooding on the Ganges-Brahmaputra temporarily displaced over 20 million in Bangladesh.

Human Manipulation of Rivers

Rivers contain only a minuscule portion of the total water on earth at any given time, but along with lakes, aquifers, and springs they are the principal sources of fresh water for humans as well as for many plants and animals. Rivers are therefore closely associated with the emergence of settled agriculture, irrigated crops, and early urban life. The great civilizations of Mesopotamia (literally “the land between the rivers”) from Sumer to Babylonia emerged beginning around 4500 BCE along the Tigris and Euphrates floodplains of modern-day Iraq. Egypt, as the Greek historian Herodotus once famously noted, was “the gift of the Nile.” The Huang spawned early Chinese civilization, just as the Indus produced the first cultures of southwest Asia, and the river valleys of coastal Peru shaped urban life in the Andes. “All the great historic cultures,” the geographer Lewis Mumford noted with only slight exaggeration, “have thriven through the movement of men and institutions and inventions and goods along the natural highway of a great river” (Mc- Cully 1996, 9).

For most of human history, river manipulation was slight, consisting mostly of diverting or impounding a portion of a river’s water for the purpose of irrigating crops. Even modest modifications of this type, however, can have severe environmental consequences. In arid regions, salinization is a common problem. Unless properly drained, irrigated fields slowly accumulate the minuscule amounts of dissolved salts naturally found in soil and water. Over time this salt buildup will eventually render the fields incapable of growing most crop species. Siltation is a common problem caused when farmers and pastoralists deforest or overgraze river valleys, inadvertently setting in motion excessive erosion downstream. As the silt settles to the channel bottom, it elevates the river above the landscape, making it more prone to flooding.

Ancient Roman, Muslim, and Chinese engineers possessed a sophisticated understanding of the art of hydraulics, as the still-extant aqueducts, canals, and waterworks of Rome, Baghdad, Beijing, and other Eurasian cities amply demonstrate. But river engineering as a mathematical science first emerged in Europe between 1500 and 1800 CE. The crucial breakthrough came when Italian engineers calculated the formula for determining the amount of water flowing in a river at any given time by measuring width, depth, and flow rate. Thereafter, water experts knew how to “tame” a river—that is, manipulate its banks, bed, and velocity in order to contain floods, reclaim land, and promote navigation—with a greater degree of precision, and therefore a higher chance of success, than had previously been feasible.

Today’s methods for controlling rivers are remarkably similar to those employed in the past—chiefly the construction of dams and weirs, the reinforcement of banks, and the straightening (and often widening) of channels—but materials and techniques have improved greatly over the past two centuries. Modern dams are designed to store water, regulate minimum channel depth (usually in connection with a lock), generate electricity, or perform all three tasks. Reinforced banks help keep the water in a designated channel, thereby reducing the frequency of flooding and opening up former floodplain for agricultural, urban, industrial, and other human uses. Channel straightening gives a river a steeper gradient and thus a faster discharge rate; by reducing the total length of a river, it also facilitates the transportation of goods between ports. Collectively, these engineering methods transform a mercurial and free-flowing stream (a “floodplain river”) into a predictable deliverer of energy, goods, and water (a “reservoir river”). Nowadays, the Nile and Yangzi produce kilowatts for industries and cities, just as the Mississippi and Rhine transport freight for companies and consumers, and the Colorado and Rio Grande deliver water for farmers and homeowners.

Environmental Consequences of Hydraulic Engineering

River engineering fosters bank-side economic growth by opening up arable land, reducing floods, promoting trade, and generating electricity, but it also has a disruptive impact on riverine environments. The problems can be divided into two interrelated types: those that compromise the purity of the water in the channel (water pollution) and those that reduce the amount of living space in its channel and floodplain (habitat loss). Both typically result in a reduction in a river’s biodiversity.

River Pollution

Water pollutants can be divided into three broad categories: nutrient-based, chemical, and thermal. The most common nutrient-based pollutants are fecal matter from untreated human sewage and agricultural runoff from phosphorus and nitrogen fertilizers. When introduced into a lake or river, these organic substances serve as food for phytoplankton (free-floating algae), which are huge consumers of the water’s dissolved oxygen. If the river moves slowly, and if the “algal blooms” are large or frequent enough, the river will gradually become eutrophified (oxygen-depleted), with negative consequences for other organisms that require dissolved oxygen for respiration. The Po and Ganges are examples of sewage-fertilizer rivers.

The most pernicious chemical pollutants include heavy metals (zinc, copper, chromium, lead, cadmium, mercury, and arsenic), and chlorinated hydrocarbons such as polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT). These substances bioaccumulate; that is, they pass unmetabolized from simple organisms to more complex organisms, magnifying in concentration as they move up the food chain. The Mersey, Rhine, Hudson, Ohio, and Donets are all examples of industrial-chemical rivers.

Thermal pollution is a problem on rivers that have numerous nuclear-, coal-, or oil-generated power plants on their banks. The heated wastewater from the plant-cooling facilities artificially raises the water temperature, and the higher temperature in turn affects the type of species capable of living in the streambed. The Rhone and Rhine are examples of thermal rivers.

Most of the world’s manipulated rivers can loosely be labeled “agricultural” since the majority of engineering projects are geared toward land reclamation (flood control) and the lion’s share of dam water is still utilized for the purpose of irrigating crops. As industrialization spreads globally, however, chemical pollutants are increasingly becoming the single greatest threat to river systems; indeed, few rivers remain completely free of any trace of industrial contaminants. Today, the cleanliness or filth of a river is often more determined by the average income of the humans who live on its banks than by the number of farms and factories in its watershed. Wealthy nations have invested in urban and industrial sanitation plants over the past fifty years, and water quality has correspondingly improved. Poorer countries, unable to afford these techno-fixes, have seen their rivers continue to deteriorate.

Habitat Loss

Water pollution compromises a river’s biological robustness by killing off organisms and by creating an unfavorable environment for nourishment and reproduction. But it is the engineering projects themselves that account for most of the habitat loss on rivers and are thus primarily responsible for the drop in biodiversity. Natural (“untamed”) rivers contain an abundance of diverse biological niches: headwaters and tributaries, main and secondary channels, deep pools and islands, banks and bed, and marshes and backwaters. Channels provide longitudinal passageways along which organisms travel, while river edges provide access routes to the adjacent marshes and floodplains where many organisms find their nourishment and reproduction sites. Floodplains nurture trees, shrubs, and reeds, which help stabilize the channel bank while providing shade and protection to other organisms. A river basin hosts a complex web of life, ranging from simple organisms such as fungi, bacteria, algae, and protozoa, to more complex organisms such as flatworms, roundworms, and wheel animals, and on up to mollusks, sponges, insects, fish, birds, and mammals.

Engineering alters a basin’s natural structure in ways that are detrimental to many species. Dams and weirs block a river’s longitudinal passageways, making it difficult for organisms to take full advantage of the channel’s living space. Migratory fish are particularly hard hit because their life cycles require them to move from headwaters to delta and back again. Most famously, salmon disappeared from the Columbia, Rhine, and many other rivers when dams were built on their banks. Reinforced banks have a similar impact: they sever the links between a river’s channel and its floodplain, depriving many organisms of their feeding and breeding sites. As a river loses all or part of its natural channel, bed, banks, islands, backwaters, marshes, and floodplain, it is transformed into a narrow and uniform rather than a broad and diverse biological site. Typically this results in a precipitous drop in both the number and type of species that it supports.

Aside from reducing the total amount of living space on a river, engineering can also trigger dramatic population upsurges in certain species, creating an ecological imbalance. The zebra mussel—a hearty algae eater and rapid reproducer—has migrated from its home in the Caspian Sea to the industrial rivers of America and Europe, displacing local mollusk species along the way. Similarly, after the completion of the Aswan Dam in the mid-1930s, snails infected with deadly schistosomes (parasitic worms) began to colonize the Nile’s new irrigation canals, debilitating and killing Egyptian farmers and fishermen.

Responding to environmentalists and to reformers from within their own ranks (such as Gilbert F. White), engineers have developed new and more sophisticated methods of river manipulation over the past thirty years. More attention is now paid to preserving the original river corridor as channel beds and banks are fortified and dredged. Dams and weirs are fitted (or retrofitted) with fish ladders to ease fish migration. More floodplain is left intact. In some cases, rivers have even been re-meandered and rebraided so that they better replicate the natural conditions that once prevailed on their banks. Nevertheless, the recent Three Gorges Dam project on the Yangzi—the largest dam-building project of all time—serves as a reminder that the environmentally unfriendly practices of the past are still in widespread use today.

Most climate scientists predict that global warming will have far-reaching impacts on river systems worldwide. High mountains such as the Alps and Himalayas may begin to shed their snowpack earlier each spring. Higher evaporation rates may cause some regions to experience significant alterations in their annual precipitation patterns. Warmer water temperatures may make some rivers inhospitable to salmon and other cold-water fish. Rising sea levels may partially or wholly inundate the Netherlands, Bangladesh, and other delta regions. Although the effects on rivers will vary from region to region, collectively these changes will pose unprecedented challenges to agriculture, manufacturing, urban water supplies, and wildlife conservation.

Bibliography:

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