The Amazon River and Mississippi River are the two most important rivers in the Americas. The Amazon basin is best known for draining the world’s largest and most biologically diverse rainforest regions, whereas the Mississippi is an important commercial artery for the United States. In addition, both rivers are navigable by large vessels deep into their continental interiors.
The Amazon River (Portuguese: Rio Amazonas, Spanish: Río Amazonas), which is also called Río Marañón and Rio Solimões, is the greatest river of South America. It possesses the largest drainage system in the world in terms of the volume of its flow and the area of its basin. The total length of the river—as measured from the headwaters of the Ucayali-Apurímac river system in southern Peru—is at least 6,400 km (4,000 miles), which makes it slightly shorter than the Nile River but still the equivalent of the distance from New York City to Rome. Its westernmost source is high in the Andes Mountains, within 160 km (100 miles) of the Pacific Ocean, and its mouth is in the Atlantic Ocean, on the northeastern coast of Brazil. However, both the length of the Amazon and its ultimate source have been subjects of debate since the mid-20th century, and there are those who claim that the Amazon is actually longer than the Nile.
The vast Amazon basin (Amazonia), the largest lowland in Latin America, has an area of about 7 million square km (2.7 million square miles) and is nearly twice as large as that of the Congo River, the Earth’s other great equatorial drainage system. Stretching some 2,780 km (1,725 miles) from north to south at its widest point, the basin includes the greater part of Brazil and Peru, significant parts of Colombia, Ecuador, and Bolivia, and a small area of Venezuela; roughly two-thirds of the Amazon’s main stream and by far the largest portion of its basin are within Brazil. The Tocantins-Araguaia catchment area in Pará state covers another 777,000 square km (300,000 square miles). Although considered a part of Amazonia by the Brazilian government and in popular usage, it is technically a separate system. It is estimated that about one-fifth of all the water that runs off the Earth’s surface is carried by the Amazon. The flood-stage discharge at the river’s mouth is four times that of the Congo and more than 10 times the amount carried by the Mississippi River. This immense volume of fresh water dilutes the ocean’s saltiness for more than 160 km (100 miles) from shore.
The extensive lowland areas bordering the main river and its tributaries, called várzeas (“floodplains”), are subject to annual flooding, with consequent soil enrichment; however, most of the vast basin consists of upland, well above the inundations and known as terra firme. More than two-thirds of the basin is covered by an immense rainforest, which grades into dry forest and savanna on the higher northern and southern margins and into montane forest in the Andes to the west. The Amazon Rainforest, which represents about half of the Earth’s remaining rainforest, also constitutes its single largest reserve of biological resources.
Since the later decades of the 20th century, the Amazon basin has attracted international attention because human activities have increasingly threatened the equilibrium of the forest’s highly complex ecology. Deforestation has accelerated, especially south of the Amazon River and on the piedmont outwash of the Andes, as new highways and air transport facilities have opened the basin to a tidal wave of settlers, corporations, and researchers. Significant mineral discoveries have brought further influxes of population. The ecological consequences of such developments, potentially reaching well beyond the basin and even gaining worldwide importance, have attracted considerable scientific attention.
The first European to explore the Amazon, in 1541, was the Spanish soldier Francisco de Orellana, who gave the river its name after reporting pitched battles with tribes of female warriors, whom he likened to the Amazons of Greek mythology. Although the name Amazon is conventionally employed for the entire river, in Peruvian and Brazilian nomenclature it properly is applied only to sections of it. In Peru the upper main stream (fed by numerous tributaries flowing from sources in the Andes) down to the confluence with the Ucayali River is called Marañón, and from there to the Brazilian border it is called Amazonas. In Brazil the name of the river that flows from Peru to its confluence with the Negro River is Solimões; from the Negro out to the Atlantic the river is called Amazonas.
The debate over the location of the true source of the Amazon and over the river’s precise length sharpened during the second half of the 20th century, as technological advances made it possible to explore deeper into the extremely remote locations of the Amazon’s head-streams and to more accurately measure stream lengths. Beginning in the 1950s, explorers of the region cited various mountains in Peru as possible sources, but they did so without taking precise measurements or applying hydrological research. An expedition in 1971, sponsored by the National Geographic Society, pinpointed Carruhasanta Creek, which runs off the north slope of Mount Mismi in southern Peru, as the source of the river. This location became widely accepted in the scientific community and remained so until the mid-1990s—although a Polish expedition in 1983 contended that the source of the river was actually another stream, nearby Apacheta Creek. (The Carruhasanta and Apacheta streams form the Lloqueta River, an extension of the Apurímac.)
With the introduction of Global Positioning System (GPS) technology in the 1990s, researchers again attempted to navigate the entire length of the Amazon. The American geographer Andrew Johnston of the Smithsonian Institution’s Air and Space Museum in Washington, D.C., employed GPS gear to explore the various Andean rivers that flow into the Amazon. Using the definition of the river’s source as being the farthest point from which water could flow into the ocean and where that water flows year-round (thereby eliminating those rivers that freeze in winter), he concluded that the source was Carruhasanta Creek on Mount Mismi.
By the early 21st century, advanced satellite-imagery technology was allowing researchers to match the river’s dimensions even more precisely. In 2007 an expedition that included members of Brazil’s National Institute for Space Research and other organizations traveled to the region of Carruhasanta and Apacheta creeks in an attempt to determine which of the two was the “true” source of the Amazon. Their data revealed that Apacheta was 10 km (6 miles) longer than Carruhasanta and carries water year-round, and they concluded that Apacheta Creek was indeed the source of the Amazon River. The team then proceeded to measure the river’s length. As part of this process, they had to determine from which of the Amazon’s three main outlets to the sea to begin the measurement—the Northern or Southern channels, which flow north of Marajó Island, or Breves Channel, which flows southward around the western edge of the island to join the Pará River estuary along the southern coast of the island. They chose to use the southern channel and estuary, because that constituted the longest distance from the source of the river to the ocean (at Marajó Bay). According to their calculations, the southern outlet lengthened the river by 353 km (219 miles). Their final measurement for the length of the Amazon—from Apacheta Creek to the mouth of Marajó Bay—was about 6,992 km (4,345 miles).
This team of researchers, using the same technology and methodology, then measured the length of the Nile River, which they determined to be about 6,853 km (4,258 miles). That value was some 200 km (125 miles) longer than previous calculations for the Nile but nearly 145 km (90 miles) shorter than the length the group gave for the Amazon. These measurements infer that the Amazon may be recognized as the world’s longest river, supplanting the Nile. However, a river like the Amazon has a highly complex and variable streambed—made more so by seasonal climatic factors—which complicates the process of obtaining an accurate measurement. Thus, the final length of the river remains open to interpretation and continued debate.
The Amazon basin is a great structural depression, a subsidence trough that has been filling with immense quantities of sediment of Cenozoic age (i.e., dating from about the past 65.5 million years). This depression, which flares out to its greatest dimension in the Amazon’s upper reaches, lies between two old and relatively low crystalline plateaus, the rugged Guiana Highlands to the north and the lower Brazilian Highlands (lying somewhat farther from the main river) to the south. The Amazon basin was occupied by a great freshwater sea during the Pliocene Epoch (5.3 million to 2.6 million years ago). Sometime during the Pleistocene Epoch (about 2.6 million to 11,700 years ago) an outlet to the Atlantic was established, and the great river and its tributaries became deeply entrenched in the former Pliocene seafloor.
The modern Amazon and its tributaries occupy a vast system of drowned valleys that have been filled with alluvium. With the rise in sea level that followed the melting of the Pleistocene glaciers, the steep-sided canyons that had been eroded into the Pliocene surface during the period of lower sea levels were gradually flooded. In the upper part of the basin—in eastern Colombia, Ecuador, Peru, and Bolivia—more-recent outwash from the Andes has covered many of the older surfaces.
The Amazon River’s main outlets are the two channels north of Marajó Island, a lowland somewhat larger in size than Denmark, through a cluster of half-submerged islets and shallow sandbanks. There the mouth of the river is 64 km (40 miles) wide. The port city of Belém, Braz., is on the deep water of the Pará River estuary south of Marajó. The Pará is fed chiefly by the Tocantins River, which enters the Pará southwest of Belém. The port city’s link with the main Amazon channel is either north along the ocean frontage of Marajó or following the deep but narrow furos (channels) of Breves that bound the island on the west and southwest and link the Pará River with the Amazon. There are more than one thousand tributaries of the Amazon that flow into it from the Guiana Highlands, the Brazilian Highlands, and the Andes. Six of these tributaries—the Japurá (Caquetá in Colombia), Juruá, Madeira, Negro, Purus, and Xingu rivers—are each more than 1,600 km (1,000 miles) long. The Madeira River exceeds 3,200 km (2,000 miles) from source to mouth. The largest oceangoing ships can ascend the river 1,600 km (1,000 miles) to the city of Manaus, Braz., while lesser freight and passenger vessels can reach Iquitos, Peru, 2,090 km (1,300 miles) farther upstream, at any time of year.
The sedimentary axis of the Amazon basin comprises two distinct groups of landforms: the várzea, or floodplain of alluvium of Holocene age (i.e., up to about 11,700 years old), and the terra firme, or upland surfaces of Pliocene and Pleistocene materials (those from 11,700 to 5.3 million years old) that lie well above the highest flood level. The floodplain of the main river is characteristically 19 to 50 km (12 to 30 miles) wide. It is bounded irregularly by low bluffs 6 to 30 metres (20 to 100 feet) high, beyond which the older, undulating upland extends both north and south to the horizon. Occasionally these bluffs are undercut by the river as it swings to and fro across the alluvium, producing the terra caída, or “fallen land,” so often described by Amazon travelers. At the city of Óbidos, Braz., where the river width is some 2 km (1.25 miles), a low range of relatively hard rock narrows the otherwise broad floodplain.
The streams that rise in the ancient crystalline highlands are classified as either blackwater (Jari, Negro, and Tocantins-Araguaia) or clearwater (Trombetas, Xingu, and Tapajós). The blackwater tributaries have higher levels of humic acids (which cause their dark colour) and originate in nutrient-poor, often sandy uplands, so they carry little or no silt or dissolved solids. Clearwater tributaries have a higher mineral content and lower levels of humic acids. Some rivers flow as clearwater during the rainy season and blackwater during the dry season. Where such blackwater tributaries enter the main river, they are sometimes blocked off to form funnel-shaped freshwater lakes or estuaries, as at the mouth of the Tapajós.
In contrast, the Madeira River, which joins the Amazon some 80 km (50 miles) downstream from Manaus, and its principal affluents—the Purus, Juruá, Ucayali, and Huallaga on the right or southern bank and the Japurá, Putumayo (Içá in Brazil), and Napo from the northwest—have their source in the geologically youthful and tectonically active Andes. There they pick up the heavy sediment loads that account for their whitewater designation. Where the silt-laden waters of the Amazon (Solimões in Brazil), derived from these streams, meet those of the Negro at Manaus, the darker and hence warmer and sediment-free waters of the latter tend to be overrun by those of the Amazon, creating a striking colour boundary that is erased by turbulence downstream.
The mother river, the Marañón above Iquitos, rises in the central Peruvian Andes at an elevation of 4,840 metres (15,870 feet) in a small lake in the Cordillera Huayhuash above Cerro de Pasco. The Huallaga and Ucayali, major right-bank affluents of the Marañón, originate considerably farther south. The headwaters of the deeply entrenched Apurímac and Urubamba, tributaries at the confluence of the Ucayali, reach to within 160 km (100 miles) of Lake Titicaca, the farthest of any stream in the system from the great river’s mouth.
The Negro River, the largest of all the Amazon tributaries, accounts for about one-fifth of the total discharge of the Amazon, and 40 percent of its aggregate volume is measured just below the confluence at Manaus. Its drainage area of about 756,000 square km (292,000 square miles) includes that of the Branco, its major left-bank tributary, with its source in the Guiana Highlands. Another of the Negro’s affluents, the Casiquiare, is a bifurcation of the Orinoco River that forms a link between the Amazon and the Orinoco’s drainage system. The Branco watershed, approximately coincident with the state of Roraima, includes extensive tracts of sandy, leached soils that support a grassy and stunted arboreal cover (campos). Other tributaries of the Negro, such as the Vaupés and Guainía, drain eastward from the Colombian Oriente. The river traverses some of the least populous and least disturbed parts of the Amazon basin, including several national parks, national forests, and indigenous reserves. In its lower reaches the Negro becomes broad and island-filled, reaching widths of up to 32 km (20 miles) in certain locations.
Canoe on the Negro River in the Amazon Rainforest, Amazonas state, northern Braz. Union Press/Bruce Coleman, Inc., New York
The Madeira River, the second largest affluent of the Amazon, has a discharge of perhaps two-thirds that of the Negro. Silt from its turbid waters has choked its lower valley with sediments. Where it joins the Amazon below Manaus, it has contributed to the formation of the 320-km- (200-mile-) long island of Tupinambarana. Beyond its first cataract, 970 km (600 miles) up the river, its three major affluents—the Madre de Dios, the Beni, and the Mamoré—provide access to the rubber-rich forests of the Bolivian Oriente. The meandering Purus and Juruá rivers that flank the Madeira on the west are also important tributaries that lead into those forests. Mamoré’s tributary, the Guaporé, opens up to the Mato Grosso Plateau.
Most of the estimated 1.3 million tons of sediment that the Amazon pours daily into the sea is transported northward by coastal currents to be deposited along the coasts of northern Brazil and French Guiana. As a consequence, the river is not building a delta. Normally, the effect of the tide is felt as far upstream as Óbidos, Braz., 970 km (600 miles) from the river’s mouth. A tidal bore called the pororoca occurs at times in the estuary, prior to spring tides. With an increasing roar, it advances upstream at speeds of 16 to 24 km (10 to 15 miles) per hour, forming a breaking wall of water from 1.5 to 4 metres (5 to 12 feet) high.
At the Óbidos narrows, the flow of the river has been measured at 216,000 cubic metres (7,628,000 cubic feet) per second, its width constricted to little more than a mile. Here the average depth of the channel below the mean watermark is more than 60 metres (200 feet), well below sea level; in most of the Brazilian part of the river its depth exceeds 45 metres (150 feet). Its gradient is extraordinarily slight. At the Peruvian border, some 3,200 km (2,000 miles) from the Atlantic, the elevation above sea level is less than 90 metres (300 feet). The maximum free width (without islands) of the river’s permanent bed is 14 km (8.5 miles), upstream from the mouth of the Xingu. During great floods, however, when the river completely fills the floodplain, it spreads out in a band some 55 km (35 miles) wide or more. The average velocity of the Amazon is about 2.4 km (1.5 miles) per hour, a speed that increases considerably at flood time.
The rise and fall of the water is controlled by events external to the floodplain. The floods of the Amazon are not disasters but rather distinctive, anticipated events. Their marked regularity and the gradualness of the change in water level are the result of the enormous size of the basin, the gentle gradient, and the great temporary storage capacity of both the floodplain and the estuaries of the river’s tributaries. The upper course of the Amazon has two annual floods, and the river is subject to the alternate influence of the tributaries that descend from the Peruvian Andes (where rains fall from October to January) and from the Ecuadoran Andes (where rains fall from March to July). This pattern of alternation disappears farther downstream, as the two seasons of high flow gradually merge into a single one. Thus, the rise of the river progresses slowly downstream in a gigantic wave from November to June, and then the waters recede until the end of October. The flood levels can reach from 12 to 15 metres (40 to 50 feet) above low river.
The climate of Amazonia is warm, rainy, and humid. The lengths of day and night are equal on the Equator (which runs only slightly north of the river), and the usually clear nights favour relatively rapid radiation of the heat received from the sun during the 12-hour day. There is a greater difference between daytime and midnight temperatures than between the warmest and coolest months. Hence, night can be considered the winter of the Amazon. At Manaus the average daily temperature is about 32 °C (90 °F) in September and 24 °C (75 °F) in April, but the humidity is consistently high and often oppressive. During the winter months of the Southern Hemisphere, a powerful south-polar air mass occasionally drifts northward into the Amazon region, causing a sharp drop in temperature, known locally as a friagem, when the mercury may drop to about 14 °C (57 °F). At any time of the year, several days of heavy rain can be succeeded by clear, sunny days and fresh, cool nights with relatively low humidity. In the lower reaches of the river basin, cooling trade winds blow most of the year.
The main influx of atmospheric water vapour into the basin comes from the east. About half of the precipitation that falls originates from the Atlantic Ocean, and the other half comes from evapotranspiration from the tropical forest and associated convectional storms. Rainfall in the lowlands typically ranges from 150 to 300 cm (60 to 120 inches) annually in the central Amazon basin (e.g., Manaus). On the eastern and northwestern margins of the basin, rainfall occurs year-round, whereas in the central part there is a definite drier period, usually from June to November. Manaus has experienced as many as 60 consecutive days without rain. Moreover, in 2005 the Manaus region experienced a devastating drought, which caused parts of the river to dry up, making transportation difficult, depleting drinking supplies, and leaving millions of rotting fish in the riverbed. Such extreme periods of drought are uncommon to the Manaus region, but fluctuations in the river’s level—thought to be related to climatic events and continued deforestation in the area—have continued to be of concern. The dry season is not sufficiently intense to arrest plant growth, but it may facilitate the onset and spread of fires, whether arsonous or natural. To the west the Andes form a natural barrier that prevents most of the water vapour from leaving the basin. The influence of mountains on rainfall is indicated by the high levels of precipitation in the upper piedmont and by the cloud-steeped Andean flanks, which feed the rivers that form a large part of the Amazon system. The highest amounts of precipitation, up to 350 cm (140 inches), are recorded in the upper Putumayo along the Colombian border.
Along the southern margin of the Amazon basin, the climate grades into that of west-central Brazil, with a distinct dry season during the Southern Hemispheric winter. As elevations increase in the Andes, temperatures fall significantly.
After the Amazon, the Paraná River (Portuguese: Rio Paraná, Spanish: Río Paraná) is South America’s longest river. It begins on the plateau of southeast-central Brazil and flows generally south to the point where, after a course of 4,880 km (3,032 miles), it joins the Uruguay River to form the extensive Río de la Plata estuary of the Atlantic Ocean.
The Paraná River’s drainage basin, with an area of about 2.8 million square km (1,081,000 square miles), includes the greater part of southeastern Brazil, Paraguay, southeastern Bolivia, and northern Argentina. From its origin at the confluence of the Grande and Paranaíba rivers to its junction with the Paraguay River, the river is known as the Alto (Upper) Paraná. This upper course has three important tributaries, namely the Tietê, the Paranapanema, and the Iguaçu, all three having their sources near the Atlantic coast in southeastern Brazil. The Alto Paraná’s passage through the mountains was formerly marked by the Guaíra Falls. This series of massive waterfalls was completely submerged in the early 1980s by the reservoir of the newly built Itaipú dam complex, which spans the Alto Paraná.
From its confluence with the Iguaçu River to its junction with the Paraguay River, the Alto Paraná continues as the frontier between Paraguay and Argentina. When it is joined by the Paraguay, it becomes the lower Paraná and commences to flow only through Argentine territory. Near Santa Fé, the lower Paraná receives its last considerable tributary, the Salado River. Between Santa Fé and Rosario the delta of the Paraná begins to form, being 18 km (11 miles) wide at its upper end and roughly 65 km (40 miles) wide at its lower end. Within the delta the river divides again and again into distributary branches, the most important being the last two channels formed, the Paraná Guazú and the Paraná de las Palmas.
Itaipú Dam on the Upper Paraná River, north of Ciudad del Este, Para. Vieira de Queiroz—TYBA/Agencia Fotografica
The volume of the lower Paraná River is dependent on the amount of water that it receives from the Paraguay River, which provides about 25 percent of the total; the Paranā’s annual average discharge is 17,293 cubic metres per second (610,700 cubic feet per second). The basin of the Alto Paraná has a hot and humid climate year round, with dry winters and rainy summers. The climate of the middle and lower basins ranges from subtropical in the north to temperate humid in the south, with less plentiful rainfall. The Alto Paraná has two zones of vegetation, forests to the east and savanna to the west. Forests continue along the Paraná downstream to Corrientes, where the savanna begins to dominate both banks. The Paraná River has a rich and varied animal life that includes many species of edible fish. Much of the Paraná basin is economically unexploited. The main dam of the huge Itaipú project on the Paraná River was completed in 1982 and had a power generating capacity of 12,600 megawatts. The Yacyretá Dam on the lower Paraná River began operation in 1994. The lower river is a transport route for agricultural products, manufactured goods, and petroleum products, and its waters are used for irrigation of the adjacent farmlands.
The vast Amazonian forest vegetation appears extremely lush, leading to the erroneous conclusion that the underlying soil must be extremely fertile. In fact, the nutrients in the system are locked up in the vegetation, including roots and surface litter, and are continuously recycled through leaf fall and decay. Generally, the soils above flood level are well-drained, porous, and of variable structure. Often they are sandy and of low natural fertility because of their lack of phosphate, nitrogen, and potash and their high acidity. Small areas are underlain with basaltic and diabasic rocks, with reddish soils (terra roxa) of considerable natural fertility. The terra preta dos Indios (“black earth of the Indians”) is another localized and superior soil type, created by past settlement activity.
The agricultural potential of the annually flooded várzea areas is great. Their soils do not lack nutrients, because they are rejuvenated each year by the deposit of fertile silt left as the waters recede, but their usage for agricultural purposes is limited by the periodic inundations. It is estimated that these valuable soils occupy some 65,000 square km (25,000 square miles).
In the early days the Amazon River was the only means of access into the forest. Francisco de Orellana descended the main course of the Amazon from the Ecuadoran and Peruvian Andes to the Atlantic in 1541–42. Nearly a century later, Pedro Teixeira went from Belém, Braz., to Quito, Ecua., and the region increasingly became known through the explorations of the Portuguese. In 1743 the French naturalist Charles-Marie de La Condamine made a raft trip down the Amazon, during which he made geographic and ethnographic observations of the basin.
At the outset of the 19th century, the German explorer Alexander von Humboldt confirmed the connection between the Amazon and Orinoco systems through the Casiquiare River. The English naturalist H.W. Bates spent time along the Amazon in 1848–59, collecting thousands of species of animals. His book The Naturalist on the River Amazons, originally published in two volumes in 1863, is still regarded as one of the great classics on the Amazon River. An official expedition was sent from the United States to Amazonia in the mid-19th century. In 1854 in Washington, D.C., William Lewis Herndon published the report that he and Lardner Gibbon—both lieutenants in the U.S. Navy—had made to Congress under the title of Exploration of the Valley of the Amazon.
The period since 1900 has been one of numerous exploratory and scientific expeditions. In 1913–14 U.S. Pres. Theodore Roosevelt and Brazilian Col. Cândido Rondon headed an expedition that explored a tributary of the Madeira and made natural history collections and observations. A party sponsored by Harvard University’s Institute of Geographical Exploration did important scientific work in the years 1910–24. The American Geographical Society compiled data and published detailed maps of this vast region.
Since World War II the international scientific community has been increasingly attracted to Amazonia. British, French, German, Japanese, and North American groups have carried out detailed biophysical and cultural surveys; a large number of international workshops, conferences, and symposia on Amazonian problems have been held. The Amazon Cooperation Treaty, signed in Brasília in 1978 by representatives of all the basin’s countries, pledged the signatories to a coordinated development of the region on sound ecological principles. (In 1995 those countries created the Amazon Cooperation Treaty Organization to strengthen and better implement the treaty goals.) Brazilian scientists have also contributed significant research on issues concerning the area. Particularly important has been the work of the National Institute of Amazonian Research (INPA) at Manaus, the Goeldi Museum in Belém, and the National Institute for Space Research in São José dos Campos.
The Mississippi River is the largest river of North America, draining with its major tributaries an area of approximately 3.1 million square km (1.2 million square miles), or about one-eighth of the entire continent. The Mississippi River lies entirely in the United States. Rising in Lake Itasca in Minnesota, it flows almost due south across the continental interior, collecting the waters of its major tributaries, the Missouri River (to the west) and the Ohio River (to the east), approximately halfway along its journey to the Gulf of Mexico through a vast delta southeast of New Orleans, a total distance of 3,780 km (2,350 miles) from its source. With its tributaries, the Mississippi drains all or part of 31 U.S. states and two Canadian provinces.
As the central river artery of a highly industrialized nation, the Mississippi River has become one of the busiest commercial waterways in the world, and, as the unruly neighbour of some of the continent’s richest farmland, it has been subjected to a remarkable degree of human control and modification. Furthermore, the river’s unique contribution to the history and literature of the United States has woven it like a bright thread through the folklore and national consciousness of North America, linking the names of two U.S. presidents—Abraham Lincoln and Ulysses S. Grant—with that of the celebrated author Mark Twain.
Although the Mississippi can be ranked as the fourth longest river in the world by adding the length of the Missouri-Jefferson (Red Rock) system to the Mississippi downstream of the Missouri-Mississippi confluence—for a combined length of 5,970 km (3,710 miles)—the 3,780-km (2,350-mile) length of the Mississippi proper is comfortably exceeded by 19 other rivers. In volume of discharge, however, the Mississippi’s rate of roughly 17,000 cubic metres (600,000 cubic feet) per second is the eighth greatest in the world.
On the basis of physical characteristics, the Mississippi River can be divided into four distinct reaches, or sections. In its headwaters, from the source to the head of navigation at St. Paul, Minn., the Mississippi is a clear, fresh stream winding its unassuming way through low countryside dotted with lakes and marshes. The upper Mississippi reach extends from St. Paul to the mouth of the Missouri River near St. Louis, Mo. Flowing past steep limestone bluffs and receiving water from tributaries in Minnesota, Wisconsin, Illinois, and Iowa, the river in this segment assumes the character that led Algonquian-speaking Indians to name it the “Father of Waters” (literally misi, “big”; sipi, “water”). Below the Missouri River junction, the middle Mississippi follows a 320-km (200-mile) course to the mouth of the Ohio River. The turbulent, cloudy-to-muddy, and flotsam-laden Missouri, especially when in flood, adds impetus as well as enormous quantities of silt to the clearer Mississippi. Beyond the confluence with the Ohio at Cairo, Ill., the lower Mississippi attains its full grandeur. Where these two mighty rivers meet, the Ohio is actually the larger. Thus, below the Ohio confluence the Mississippi swells to more than twice the size it is above. Often a mile and a half from bank to bank, the lower Mississippi becomes a brown, lazy river, descending with deceptive quiet toward the Gulf of Mexico.
To geographers, the lower Mississippi has long been a classic example of a meandering alluvial river. That is, the channel loops and curls extravagantly along its floodplain, leaving behind meander scars, cutoffs, oxbow lakes, and swampy backwaters. More poetically, Mark Twain compared its shape to “a long, pliant apple-paring.” Today the sunlight glittering on the twisted ribbon of water remains one of the most distinctive landmarks of a transcontinental flight. Now curbed largely by an elaborate system of embankments (levees), dams, and spillways, this lower section of the Mississippi was the golden, sometimes treacherous, highway for the renowned Mississippi steamboats, those “palaces on paddle wheels” that so fired the public imagination. From the explosive master pilot Horace Bixby, portrayed by Mark Twain, to the nostalgic lyrics of Oscar Hammerstein’s song “Ol’ Man River,” the creations of that era on the Mississippi have added colour to America’s heritage.
The geology and physical geography of the Mississippi drainage area are essentially those of the Interior Lowlands and Great Plains of North America. Fringes also touch upon the Rocky and Appalachian mountains and upon the rim of the Canadian (Laurentian) Shield to the north. The focus of the system, the floodplain of the lower Mississippi, is of particular interest in that the geology and physical geography of the region are of the river’s own making. Like a huge funnel, the river has taken sediment and debris from contributory areas near the lip of the funnel and deposited much of the product in the alluvial plain of the funnel’s spout, illustrating the interdependence of the entire Mississippi system.
The most significant contributory area in recent times has been to the west of the river. Rising in western uplands, notably in the foothills of the Rockies, rivers such as the Red, Arkansas, Kansas, Platte, and Missouri remove considerable silt loads from the rolling expanses of the Great Plains. These tributaries meander and braid across a wide, gently sloping mantle of unconsolidated materials, laid down over rock beds of the Cretaceous Period (i.e., about 100 million years old), toward the “Father of Waters.” Precipitation in these western areas is light to moderate, usually less than 6.35 cm (25 inches) per year, but, because at least 70 percent of this precipitation falls as rain between April and September, the erosive capability of the rivers is enhanced (runoff from winter snowmelt is more gradual than from rainstorms). The sandy sediments, moreover, offer little resistance to erosion, so that many of these rivers are only braided in their courses.
The Mississippi’s eastern contributory rivers drain the well-watered Appalachian Mountain system. Most of this group, including the Kentucky, Green, Cumberland, and Tennessee rivers, flows via well-defined valleys into the Ohio and thence into the Mississippi. The erosive capacity of these rivers varies in relation to the geologic structure of their basins. These consist of harder rocks in the higher elevations and a softer sill of limestone of the Late Carboniferous Period (i.e., about 300 million years of age), lying below the 305 metres (1,000-foot) elevation line between the Ohio and Tennessee rivers and in the glaciated area of the Ohio’s right-bank tributaries.
The third contributory area of the Mississippi also differs from the other two. The upper Mississippi gathers its strength in a region marked by glacial action. After the great ice sheets of the Wisconsin Glaciation had put down layers of debris across much of Minnesota, Wisconsin, northern Illinois, and northern Iowa, huge quantities of meltwater flowed south, washing channels through this debris. Today the upper Mississippi and its tributaries, the Wisconsin, St. Croix, Rock, and Illinois rivers, all trace the lines of these former sluiceways.
Pouring southward, the glacial meltwaters were joined by the proto-Missouri and Ohio rivers. The combined waters then enlarged the great north-south trough along which the lower Mississippi now flows. Some 1,600 km (1,000 miles) long, this trough is 40 to 320 km (25 to 200 miles) wide and bounded by escarpments rising up to 61 metres (200 feet) above the valley floor. Geologic studies have revealed that the floor of the glacial trough was later buried by a deep layer of material washed out from an ice sheet and dumped to a thickness of about 30 to 90 metres (100 to 300 feet) in the central section.
The Mississippi’s delta is an even more striking monument to the river’s constructive work. There, at the tip of the drainage funnel, millions of years of sedimentation have spilled out across the floor of the Gulf of Mexico, forming cones of sediment that total 480 km (300 miles) in radius and 77,800 square km (30,000 square miles) in area. The surface expression of the many sub-deltas is the Mississippi delta, with an area exceeding 28,500 square km (11,000 square miles). Stretching its distributaries into the gulf, the Mississippi once delivered some 220 million tons of sediment there each year, most of it as silt. Today, however, much of this silt is captured behind upstream dams, causing the delta to erode and shrink in area. Compounding this problem are the many hundreds of miles of levees (walls that limit flooding) along the river’s banks, which trap silt in the channel proper. This is especially damaging in the delta, where annual silt additions from flooding help to keep it from being eroded by waves.
During winter, mean monthly temperatures in the Mississippi basin range from 13 °C (55 °F) in subtropical southern Louisiana to -12 °C (10 °F) in subarctic northern Minnesota. Mean monthly temperatures in summer range from 28 °C (82 °F) in Louisiana to 21 °C (70 °F) in Minnesota.
Precipitation sources are low-level moisture from the Gulf of Mexico and some low-level and high-level moisture from the Pacific Ocean. Winter and spring precipitation occurs in the vicinity of easterly and southerly fronts and storms. Average monthly precipitation in winter ranges from 13 cm (5 inches) or more in the south to more than 7.5 cm (3 inches) over much of the Ohio River basin to less than one inch over the western and northern Great Plains. Summer and early autumn rainfall occurs mostly as showers and isolated thunderstorms and weaker frontal storms. Average monthly rainfall ranges from 15 cm (6 inches) in southern Louisiana and over the mountains of Tennessee and North Carolina to only 5 to 7.5 cm (2 to 3 inches) over the Great Plains.
The climate is humid over the eastern half of the basin, with large quantities of winter and spring runoff generated over the Tennessee, Ohio, and southern Mississippi river basins. A north-south band of subhumid climates, neither fully humid nor semiarid, extends from central Texas northward to eastern North Dakota. To the west are the semiarid climates of the Great Plains, and along the Rocky Mountain crests an alpine climate prevails, in which winter snowfalls are released as spring and early summer meltwater runoff.
It is not surprising that the hydrology of so powerful a river as the Mississippi has been the subject of intense study. In the 19th century Mark Twain described with considerable wit how the pilots of the Mississippi paddle wheelers banded together to run a common information service about changing conditions along the channel. Today the Mississippi River Commission is responsible for river work and considers it worthwhile to maintain a working scale model of the river so that its engineers can test new plans in miniature before embarking on expensive, full-scale projects. Indeed, by the 1920s it was generally believed that enough was known about the river’s hydrology and enough control structures had been built to have tamed the river. Then in 1927 came the most disastrous flood in the recorded history of the lower Mississippi valley. More than 59,500 square km (23,000 square miles) of land flooded. Communications, including roads and rail and telephone services, were cut in many places. Farms, factories, and whole towns went temporarily underwater. An immense amount of property was damaged, and at least 250 people lost their lives. The river engineers took another look at the hydrology of the Mississippi.
Since the freak conditions of 1927, the mean discharge of water into the lower Mississippi by its major tributaries has been carefully monitored. The mean discharge of the main river at Vicksburg, Miss., is calculated at about 16,100 cubic metres (570,000 cubic feet) per second. About 220 km (135 miles) downriver from Vicksburg, approximately 25 percent of the sediment and water discharge of the river is diverted into the Atchafalaya River through the Old River Complex (Old River Control Structures). These statistics, however, conceal all-important variations in river flow linked with the fluctuating state of the Mississippi’s larger tributaries.
Broadly speaking, the western tributaries have the most-irregular flow regimes. They reach a spring or early summer peak that is up to three or four times as great as their winter contribution. The upper Mississippi and its tributaries reach their maximum flow about the same time (March–June), when melting snows are followed by early summer rains. The winter runoff from this area, however, is also substantial. The crest of the Ohio’s flow occurs slightly earlier. At Metropolis, Ill., just above the confluence with the Mississippi, the greatest monthly discharge is usually recorded in March, at which time the Ohio may be providing more than three-fifths of the water being monitored past Vicksburg in the lower river.
Thus, the Ohio is chiefly responsible for the lower Mississippi flood situations, which may be aggravated by such factors as early rains in the Great Plains, a sudden hot spell in early spring that melts the northern snows, and heavy downpours throughout the lower valley. Under such conditions the lower river will rise over its banks and put pressure against its man-made levees. Tributaries will back up and form lakes on the far side of these same levees. The current, which normally runs no more than 2 to 3.5 knots (2.5 to 4 miles per hour), may then double at constricted points along the main channel. Thus, for example, the monitoring station at Vicksburg, which at low water in 1936 recorded as little as 2,656 cubic metres (93,800 cubic feet) per second, measured 58,300 cubic metres (2.06 million cubic feet) per second at high-water stage the following year.
In late spring and early summer of 1993 another inevitable yet inconceivably large flood occurred on the Mississippi, this time confined to the parts of the river above its confluence with the Ohio (which was not in flood). Among the worst-hit rivers were the lower reaches of the Missouri, the Des Moines and Raccoon rivers in Iowa, and the Mississippi between the Wisconsin-Illinois border and Cape Girardeau, Mo. The floods were set off by persistent rains in this region. For the first time in recorded history, the Mississippi and the Missouri flooded at the same time—despite the 29 dams on the Mississippi and the 36 giant reservoirs on their tributaries. The Raccoon River in Des Moines crested at 2.1 metres (seven feet) above the previous high, which constituted a 500-year flood event (a flood so large that it occurs, statistically, only about once every five hundred years; or that it has a one in five hundred chance of happening in any year).
The Missouri River in Montana. Travel Montana
The Missouri River is the longest tributary of the Mississippi River, formed by the confluence of the Jefferson, Madison, and Gallatin rivers in the Rocky Mountain area of southwestern Montana (Gallatin county), U.S., about 1,200 metres (4,000 feet) above sea level. The Missouri flows eastward to central North Dakota, where it turns southward across South Dakota to form a section of the South Dakota–Nebraska boundary, the Nebraska–Iowa boundary, the Nebraska–Missouri boundary, and the northern section of the Kansas–Missouri boundary. Then it meanders eastward across central Missouri to join the Mississippi River, about 16 km (10 miles) north of St. Louis, after traveling a course of 3,726 km (2,315 miles).
Its drainage basin occupies about 1,371,100 square km (529,400 square miles) of the Great Plains, of which 16,840 square km (2,550 square miles) are in Canada. Elevations within its basin are extreme: from 4,300 metres (14,000 feet) above sea level in the Rockies near the Continental Divide to 120 metres (400 feet) where it joins the Mississippi. The flow of the Missouri and of most of its tributaries is exceedingly varied—the minimum flow being 120 cubic metres (4,200 cubic feet) per second and the maximum 25,500 cubic metres (900,000 cubic feet) per second. With unprotected slopes and with such violent fluctuations in flow, erosion and silting are major problems.
Chief tributaries include the Cheyenne, Kansas, Niobrara, Osage, Platte, and Yellowstone rivers, flowing in on the south and west sides, and the James and Milk rivers, entering from the north. Other tributaries are the Bad, Blackwater, Cannonball, Gasconade, Grand, Heart, Judith, Knife, Little Missouri, Moreau, Musselshell, and White rivers, which enter from the south and west. The Big Sioux, the Chariton, Little Platte, Marias, Sun, and Teton rivers enter from the north and east.
The Missouri was named Peki-tan-oui on some early French maps and, later, Oumessourit. It has been nicknamed “Big Muddy” because of the amount of solid matter it carries in suspension. Its mouth was discovered in 1673 by the French explorers Jacques Marquette and Louis Jolliet while they were canoeing down the Mississippi River. In the early 1700s French fur traders began to navigate upstream. The first exploration of the river from its mouth to its headwaters was made in 1804–05 by Meriwether Lewis and William Clark. For many years commerce on the river was restricted to the fur trade, and the river was little used by the earliest American settlers moving west. The American Fur Company began to use steamers on the river in 1830. Steamboat traffic on the river reached its height in 1858 but began to decline in the following year with the completion of the Hannibal and St. Joseph Railway to St. Joseph, Mo.
For the first 150 years after settlement along the river, little was done to develop the Missouri as a useful waterway or as a source of irrigation and power. In 1944 the U.S. Congress authorized a comprehensive program for flood control and water-resource development in the Missouri River basin. It envisioned a system of more than one hundred dams and reservoirs on the Missouri and certain of its tributaries. Local flood protection, involving levees and bank stabilization, and a deeper river channel were provided on the Missouri itself from Sioux City, Iowa, to the Mississippi, a distance of 1,220 km (760 miles). By the time an even more ambitious plan, the Missouri River Basin program, or simply the Pick-Sloan program, was adopted in the 1950s, channel maintenance had enabled commercial barge lines to begin operating on the Missouri in 1953. The major dams built on the Missouri were Fort Peck (near Glasgow, Mont.), Garrison (N.D.), and Gavin’s Point, Fort Randall, and Oahe (S.D.). The Fort Peck Dam is one of the largest earthfill dams in the world. The entire system of dams and reservoirs has greatly reduced flooding on the Missouri and provides water to irrigate millions of acres of cropland along the main river and its tributaries. Hydroelectric installations along the river generate electricity for many communities along the river’s upper course.
The chief cities along the Missouri are Great Falls, Mont.; Williston and Bismarck, N.D.; Pierre, S.D.; Sioux City and Council Bluffs, Iowa; Omaha and Nebraska City, Neb.; Atchison, Leavenworth, and Kansas City, Kan.; and St. Joseph, Kansas City, Columbia, Jefferson City, and St. Charles, Mo.
In many parts of Iowa crops never got planted. In all, some 6 million hectares (15 million acres) flooded, and 40 federal levees and 1,043 nonfederal levees broke. This disastrous flood taught many that flood-control structures such as levees, floodwalls, and dams work for some events but fail to provide enough protection from the one-hundred-year (or larger) floods. The floods of 1993 taught many that tight and total control of rivers as large as the Mississippi is neither possible nor economically feasible. Since then, it has become clear that “living with the river” means moving homes, farmhouses, and even entire towns off the floodplains and allowing these lowland areas to flood naturally.
A variety of pollutants, derived from municipal, industrial, and agricultural sources, have been identified in the waters and sediments of the Mississippi River. Organic compounds and trace metals occur in relatively low concentrations. In addition to those naturally present in the water, they derive from industrial and municipal wastes and runoff from agricultural and urban areas. High concentrations of bacteria associated with human waste, however, have been found downstream from some cities and have been attributed to inadequately treated sewage flowing into the river. Concentrations downstream from New Orleans, for example, have been found to be many times greater than concentrations above the city. Pollutants have had little widespread effect on the composition of benthic invertebrate populations, which are indicative of changes in water quality. Water samples taken at New Orleans have shown a relatively high dissolved-oxygen content and low biochemical oxygen demand. Thus, by this index, river pollution may be said to be low.
