River

• 1. The Water That Shapes The World

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  A river is a natural stream of fresh water, moving across land or through flooded caves toward something lower. It might be an ocean, a lake, or another river waiting downhill. Some rivers never finish the journey. They run dry before the end, or flow only in certain seasons, then vanish until the water returns. About half of all waterways on Earth behave this way, never holding a steady flow through the whole year. How does water that falls as rain end up carving canyons and feeding the first cities? Why do people kneel at a river's edge to be cleansed of sin, and also build walls of soil to keep it from drowning their fields? And why are geologists now searching the surface of Mars for the ghost of a river that dried up billions of years ago? The answers begin with a single, simple fact. Every drop in every river came down from the sky.

• 2. Born From Rain

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  All water that flows in rivers must ultimately come from precipitation, the source from which the rest follows. It arrives as rainfall, as runoff sliding down a slope, as melting glaciers and snow, or as seepage from aquifers below the ground. The smaller streams that gather and feed a river are its headwaters, and together they form the river's source. Some race down the sides of mountains, fast and thin. All the uphill land that drains into a river belongs to its drainage basin, also called its watershed. A ridge of higher ground usually keeps one basin separate from the next. Water on one side of that divide goes one way, and water on the other side goes another. The Continental Divide of the Americas, running through the Rocky Mountains, makes this dramatic. Rain on the western side flows to the Pacific Ocean. Rain on the eastern side flows to the Atlantic. Not every drop reaches a river directly. Some seeps into underground aquifers and waits, stored in the soil as groundwater. Where a river sits lower than this water table, the hidden water flows back in. That quiet exchange is why rivers can keep flowing even during a drought. In summer, higher temperatures melt snow and ice in high country, and glacier melt arrives late in the season when the snow has already gone, keeping the rivers downstream supplied.

• 3. The Restless Channel

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  Gravity sets the direction, pulling every river downhill toward lower ground. A common misconception holds that rivers mostly run north to south, and that is simply not true. As they descend, they merge into larger rivers; a feeder stream is a tributary, and the meeting place is a confluence. Where a river cuts through impermeable rock, it erodes the slopes on either side. Carving into a plateau, it can open a canyon walled by cliffs. Softer rock weathers faster than hard rock, and that uneven wear can drop the riverbed suddenly, forming a waterfall where the flow tips over a vertical edge. Rivers rarely run straight, and there is a reason they bend. Any natural obstacle deflects the current, and alluvium piles against it, nudging the course aside. The flow then strikes the opposite bank, eroding it into a concave curve, which throws the water back the other way. Repeated over and over, this back-and-forth carves a meander. Their banks shift often, and the river is never quite where it was. In low, flat country near the sea, rivers spread across floodplains that flood when the water runs high. When the banks spill over, they lay down mineral-rich alluvium, feeding the soil and supporting both farming and a host of plant and animal life. Where the climate makes the flooding predictable, people speak of wet seasons and dry seasons. The deposited sediment can also build fluvial islands, which exist in almost every river.

• 4. Sediment And The Sea

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  Alluvium is the name for the sediment a river carries, the debris that rebuilds the land downstream. It comes from the river's own erosion, from material washed in by rain, and from the slow grinding of glaciers. The sand of deserts and the bars that form islands both trace back to rivers. As the water moves, it sorts the load by weight. Heavy rocks settle to the bottom, while finer sand and silt travel further before they come to rest. The sediment yield measures how much sand leaves a watershed over time, per unit of area. Ecologists watch it closely, because it reveals the health of the ecosystem, the pace of erosion, and the mark of human activity. At the very end of the journey, the mouth of a river takes several forms. A tidal river, often part of an estuary, rises and falls with the tide, and its brackish water and alluvium can drift upriver or downriver depending on the hour. Other rivers build deltas, pushing sediment into the sea until new land accumulates, shaped by the contest between the river's strength, the waves, and the tidal current. Seen from above, a delta fans out from the old coastline in a spread of triangular shapes. Not every river makes it that far. If groundwater losses or human demand siphon away too much water, the riverbed can run dry before reaching the sea, ending instead at a local low point in an endorheic basin.

• 5. Life Along The Current

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  Shredders, grazers, collectors, and predators each play a role in a river's web of life, a division drawn from the River Continuum Concept. Shredders consume organic material. Grazers, also called scrapers, feed on the algae coating rocks and plants. Collectors eat the detritus of the dead, and predators hunt the living. The river can then be read by what resources each role can find. A shady stretch under deciduous trees gets a steady fall of leaves, so shredders and collectors thrive there. Where the water grows deeper, wider, and brighter, invertebrates and varied fish appear, alongside scrapers working the algae. Further downstream, much of the energy arrives as matter already processed upstream, and predators grow more active, including fish that feed on plants, plankton, and other fish. The riparian zone is the band of land that meets the water, and it does heavy work. Its plants stabilize the banks against erosion, filter the deposited alluvium, and process nitrogen and other nutrients. Forests there also shelter animals. The flood pulse concept centers on habitats that flood seasonally, including lakes and marshes. A larger river holds more kinds of life, an idea called the fish zonation concept. Small rivers suit only small fish, while large rivers host both small and large, much like the species-area relationship in which bigger habitats carry more species. Here it is named the species-discharge relationship, tied to the river's discharge, the volume of water passing through at a given moment. A river can also be a wall. The Amazon runs so wide in places that the species on opposite banks are distinct. Some fish swim against the current to spawn. Those that travel from sea to freshwater to breed are anadromous, and those that move from rivers to the ocean are catadromous. Salmon, which are anadromous, may die in the river after spawning, returning their nutrients to the water that raised them.

• 6. Engineering The Flow

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  Four hundred million tons of sediment per year once moved down the Mississippi River, a figure that engineering has cut by 60 percent. Reservoirs trap it, levees hold it back, and revetments replace the natural banks. This is one mark of modern river engineering, a sprawling set of structures aimed at flood control, navigation, recreation, and ecosystem management. The plainest projects clear obstructions like fallen trees. Larger ones dredge a channel deeper for boats, and all of it needs constant maintenance as banks shift and floods deliver new debris. Channelization cuts off winding sections with a shorter, straighter path; on the Missouri River, this trimmed the distance to travel by 116 km. Dikes, set beneath the surface and perpendicular to the flow, speed the water in the channel's middle and help straighten it. Levees work like dams along a river's sides, built up from soil or clay to hold back floodwater, and some add floodways to steer the overflow away from farms and towns. Dams restrict the flow entirely. They raise the water upstream for navigation, generate hydroelectricity, and turn the stretch behind them into a lake or reservoir that can supply a city with drinking water. As renewable energy, hydroelectricity needs no input beyond the river itself. Dams are everywhere, with at least 75,000 taller than 6 feet in the United States, and reservoirs worldwide covering 193,500 square miles. Dam-building peaked in the 1970s, when two or three were finished every day, and has since declined, with new projects now centered in China, India, and other parts of Asia.

• 7. Cradle Of Civilization

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  The first civilizations were born on floodplains between 5,500 and 3,500 years ago, where fresh water, fertile soil, and easy transport made complex society possible. Three rose in river systems: the Sumerians in the Tigris-Euphrates, the Ancient Egyptians along the Nile, and the Indus Valley Civilization on the Indus River. Surrounding deserts left these people utterly dependent on the water, drawing them into clusters that became the first cities. They are thought to be the first to irrigate desert ground for food, and growing food at scale let people specialize, form hierarchies, and organize in new ways. Infrastructure followed close behind. The Sadd el-Kafara dam near Cairo was built on the Nile 4,500 years ago. Roman aqueducts carried water to cities, and Spanish Muslims ran mills and water wheels from the seventh century. Between the years 130 and 1492, larger dams rose in Japan, Afghanistan, and India, including 20 taller than 15 meters. Canals were cut in Egypt as early as 3000 BC, and the mechanical shadoof lifted water to higher ground. Water wheels began harnessing river energy at least 2,000 years ago, turning an axle to move water, work metal with a trip hammer, or grind grain with a millstone. In the Middle Ages, water mills spread fast, and by 1300 there were at least 10,000 in England alone. A single medieval watermill could do the work of 30 to 60 people. Yet floodplain civilizations were sometimes abandoned on a large scale, blamed at first on catastrophic floods, though lasting climate shifts toward aridity and lower river flow may have decided which ones survived.

• 8. Sacred, Contested, And Beyond Earth

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  Charon the boatman ferried the souls of the dead across the River Styx in exchange for money, in the geography of the Greek underworld. Souls judged good entered Elysium and drank from the River Lethe to forget their former lives. Rivers run through paradise in Abrahamic scripture too. In Genesis, a river rises in the Garden of Eden and splits into four, among them the Tigris and Euphrates, with two others possibly pointing to the Nile and the Ganges. The Quran describes four rivers flowing with water, milk, wine, and honey. Floods carry their own weight of meaning. The Genesis flood cleansed the Earth of human wrongdoing, an act echoed in the Epic of Gilgamesh, in Sumerian mythology, and elsewhere, and likened to the Baptism of Jesus in the Jordan River. In Norse myth the world emerges from a void into which eleven rivers flowed. Particular rivers were held sacred. Ancient Celts saw rivers as goddesses, the tears of Isis were said to cause the Nile's yearly flood personified by Hapi, and in Yoruba religion Yemoja rules the Ogun River in modern Nigeria, creating all children and fish. In Hinduism, archeological evidence points to mass ritual bathing in the Indus river valley at least 5,000 years ago, and the Ganges remains most sacred, its water said to heal and to absolve sin. Rivers also draw borders. The Lamari River in New Guinea divides the Angu from the Fore, who speak different languages and rarely mix, and 23 percent of international borders are large rivers wider than 30 meters. The Danube, once the northern edge of the Roman Empire, now separates Hungary and Slovakia. Because a river's course shifts, the Rio Grande between the United States and Mexico is governed by the International Boundary and Water Commission to fix the line and divide the water. Up to 60 percent of the fresh water nations use crosses international borders, and Egypt holds an agreement with Sudan requiring a minimum yearly volume past the Aswan Dam. The reach of rivers does not stop at this planet. Mars has no liquid water today, only permafrost ice caps and traces of vapor, yet rivers flowed there for at least 100,000 years. In Hellas Planitia, an impact crater, sedimentary rock formed 3.7 billion years ago and lava fields 3.3 billion years old, high-resolution images reveal channels and deltas, and bench-and-slope landforms mark old river erosion. On Saturn's moon Titan, the term flumen names channels that may carry liquid methane and ethane, with river valleys and seas that scientists hope to study to learn how coasts erode without any human hand.