In the lower reaches, the floodplain tends to be wide and the river channel bordered by natural levees. On well-established floodplains, cities are sometimes built behind the natural lev-ees at several feet below river level. This is the case at Vicks-burg, Mississippi, where the Mississippi River flows nearby at several feet above the height of passersby in nearby streets.
Where natural levees prevent a tributary from reaching the main river, the tributary may flow parallel to the main chan-nel for many miles before a break in the levee system allows the tributary to join the main river. Such tributaries are called yazoo streams after the Yazoo River, which runs parallel with the Mississippi River for more than 100 miles (160 km) before joining it at Vicksburg. Backswamps are marshes that form in damp depressions behind levees. Oxbow lakes—called bayous
in Louisiana and billabongs in Australia—are also common in the lower reaches of river systems.
Sooner or later, most rivers come to an end when they flow into a lake or into the sea. The river loses its forward motion and gradually releases its sediment load. The coarsest sedi-ment—usually sand—is commonly dropped right at the mouth of the river. Finer sand is dropped farther out, fol-lowed by silt and finally, farthest out, clay.
The deposited material in and around the river mouth forms a platform called a delta, across which the river chan-nel commonly breaks up into several chanchan-nels called distribu-taries. The name delta is credited to the Greek historian Herodotus (fifth century B.C.E.), who noted that Egypt’s Nile River irrigates a triangular wedge of land at its mouth. He River channels
meandering through the rain forest of the Kikori Delta, Papua New Guinea. Those channels that discharge into
the sea are called distributaries. (Gerry Ellis/Minden Pictures)
named this irrigated land the delta after the triangular shape of the Greek letter delta: D.
Over hundreds of years, as distributaries deposit their sedi-ment load, the delta gradually extends outward from the
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The classic triangular delta of the Nile River and the unusual bird’s-foot delta of the Mississippi River
mouth of the river. As a delta grows, the major flow of water shifts from one distributary to another, as first one distribu-tary and then another extends and then gradually becomes clogged with sediment. The delta of the Mississippi has grown in this way for about 150 million years. It began life somewhere in the vicinity of the junction of the present-day Ohio and Mississippi Rivers. Since then, it has advanced some 1,000 miles (1,600 km) southward, in the process creat-ing most of the land that now forms the U.S. states of Louisiana and Mississippi. Analysis of sediment deposits shows that the Mississippi delta has shifted position several times in the last 6,000 years, at various times occupying land east and west of its current position.
Not all river mouths deposit deltas. Where waves or cur-rents prevent sediment from settling close to the river mouth, the sediment deposits offshore or elsewhere and a delta does not form. This happens in the case of the Amazon (see “Amazon River,” pages 58–60) and Congo (see “Congo [Zaire] River,” pages 61–64) Rivers.
A satellite image of Upper Chesapeake Bay, in the northeastern United States.
This infrared image shows the land as predominantly red and water as various shades of blue. The Potomac River snakes from upper left to bottom right. Chesapeake Bay is the valley of the lower Susquehanna River drowned by rising sea levels since the last Ice Age. Riverborne sediment does not accumulate in the
bay to form a delta.
(Courtesy Department of Commerce/National Oceanic and Atmospheric
Administration)
The depth of water, the rate of sediment supply, and the relative strengths of river flow and coastal currents affect the form of a coastal delta. The Nile River has a classic fan-shaped (arcuate) delta. The Mississippi, by contrast, has a bird’s-foot delta, with distributaries bounded by sediment deposits, but with stretches of clear water between them. This gives the delta the appearance of a bird’s foot when viewed from the air. The Mississippi’s bird’s-foot arrangement establishes itself because the river flow is strong, the channel is kept clear by dredging, the continental shelf is shallow, and waves and currents in the Gulf of Mexico are not strong enough to redistribute the sediment more thoroughly.
Floods
Technically, a flood occurs when part of a river system rises above its banks and the water inundates some of the sur-rounding land. Flood stage is the level at which the river begins to overflow its banks.
The water level in a river system usually varies with season.
The water level rises to flood level when the input of water, either from recent precipitation or from stored, frozen water that has thawed, is much greater than the rate at which water is discharged from the river system. There is a temporary major imbalance between input and output. Water that enters the river system cannot escape quickly enough through the mouth of the main channel, so the river level rises.
In many rivers, flooding is a common feature and happens every year or almost every year. In other cases, floods are rare.
For a given river system, major floods tend to be rarer than minor floods. By studying historical records—and increasing-ly, by monitoring precipitation levels and river-height meas-urements using automatic sensing equipment—hydrologists can estimate the likelihood of flooding by entering the data into their latest computer models. Using radar sensors carried by aircraft, scientists can accurately plot the contours of the land close to the river and calculate which areas will flood under different conditions.
Floods are described in terms of how often they are likely to occur. For example, suppose a hydrologist has 100 years of data for a river and finds that 10 times in that century the river
rose three feet (0.9 m) above flood stage. Such a flood is called a 10-year flood because, on average, it is likely to occur once every 10 years. Put another way, such a flood has a one in 10 chance (or 10 percent probability) of happening in a given year. If the river rises five feet (1.5 m) above flood stage twice in the century, it is a 50-year flood, and in a given year it has a one in 50 chance (or 2 percent probability) of happening.
Such flood estimates are not predictions. Rather they are expressions of probability based on previous experience. If cli-matic or river conditions change, these estimates—based on past experience—become much less reliable. Global warming, for example, is altering patterns of precipitation and is caus-ing sea level to rise, makcaus-ing floodcaus-ing of lower reaches more likely (see “Climate change,” pages 196–199). And the con-struction of artificial levees is also altering flood patterns.
Flood defenses such as levees make small floods less likely but can make the effects of larger floods more catastrophic.
Instead of being dissipated gradually over the land as the water rises, the floodwater may breach flood defenses only when the water reaches a high level, resulting in a destructive surge that sweeps across the landscape. Flood defenses may make people assume the land on floodplains is safe because the region experiences floods much less frequently. The reduced occurrence of smaller floods may encourage develop-ers to build upon the floodplain. Such constructions on flood-plains may remain as susceptible to large, occasional floods as they were before—perhaps even more so.
The worst U.S. flood (with the exception of coastal flooding associated with Hurricane Katrina in 2005) occurred in 1993 in the middle and lower reaches of the Mississippi River. Pro-longed rain over the northern Great Plains caused extensive flooding—a 100-year flood. It killed 487 people and caused in excess of $15 billion of property damage. At St. Louis, Mis-souri, the river remained above flood stage for 144 of 183 days between April and September. Since the 1993 flood, hydrolo-gists have had to reconsider the extent to which the Mississip-pi is constrained by artificial levees (see “MississipMississip-pi River,”
pages 69–72). In response to the catastrophic flood, the popu-lation of Valmeyer, Illinois, has now relocated most of the town to higher ground near its pre-1993 location.
Across the world there are many thousands of rivers and sev-eral million lakes. Each river system and major lake has its own hydrology, ecology, and human history. This chapter gives a snapshot of eight of the world’s most important rivers and three of its lakes, chosen in part to reflect a representa-tive diversity across the major continents.
The Amazon River is the world’s most important, both in terms of size and its biological diversity. Africa’s Congo (Zaire) River, like South America’s Amazon, is a giant tropical river, but unlike the Amazon its flow is fairly regular, not highly seasonal. The Danube River is a representative exam-ple of a major European river with a temperate climatic regime. Like most major European rivers, it has been heavily affected by pollution and by activities such as damming and channel deepening and straightening, which have radically altered the river’s flow characteristics. The Ganges River is a tropical Asian river that is a lifeline for some of the world’s most densely populated regions. The Ganges is massively affected by water extraction for agriculture and is heavily degraded by pollution from untreated sewage. The Missis-sippi is in some ways North America’s counterpart to Europe’s Danube, being a heavily polluted thoroughfare car-rying busy boat traffic. Like the Danube, the Mississippi expe-riences massive fluctuations in river flow and floods severely.
The Nile River—the world’s longest—straddles tropical and warm temperate climatic regimes. It provides a classic exam-ple of the very mixed effects that damming can have on a major river system. China’s Yangtze River is, like the Ganges, a provider of irrigation water for some of the world’s most important agricultural regions, providing food for many mil-lions of local people. The Yangtze’s current dam projects will profoundly affect the river, its wildlife, and its people.