Nile River

The Nile River is an interesting and important hydrologic system. Its complex geologic evolution began with the drying up of the Mediterranean Sea about six million years ago. The Nile carries water northward across the Sahara Desert from high-rainfall regions of equatorial Africa. It nurtured one of the great civilizations of ancient times and sustains millions of people in modern Egypt.

Geography and Characteristics

The Nile River is the longest river on Earth, extending some 4,258 miles (6,853 kilometers) from Lake Victoria to the Mediterranean Sea. Only the Amazon River in South America approaches its length. The river follows a generally south-to-north path, as both the source of the White Nile in equatorial Africa and its mouth on the southern shore of the Mediterranean Sea lie within 1 degree of longitude. The Nile crosses thirty-five degrees of latitude, a distance comparable to the width of the continental United States, and flows across a greater variety of regions than any other river.

In spite of its great length and large drainage basin (approximately 1.3 million square miles or 3.3 million square kilometers, covering about 10 percent of the African continent), the Nile carries relatively little water. During the twentieth century, annual flow ranged from a low of 10 cubic miles (42 cubic kilometers) in the drought year of 1984 to a high of 29 cubic miles (120 cubic kilometers) in 1916. This comparatively small flow results from the fact that no water is added to the river for the second half of its journey to the sea, while a significant amount of the water is lost to evaporation. Most rivers merge with increasingly large streams as they approach the sea, joining their waters into an ever-swelling stream. The Nile, however, flows through the Sahara Desert, the largest and most desolate tract of land on Earth, matched only in size and desolation by the icy wastes of Antarctica.

Passage through the Sahara Desert increases the importance of Nile water while reducing its volume. Its greatest value is reached as it nears the sea, where it has nurtured agriculture in Egypt for more than five thousand years. The rich soil, near-constant sunshine, and abundant water of the Nile River valley in Egypt combine to produce one of the most productive agricultural regions of the world. The ancient civilizations of Egypt were built on this firm economic foundation. Without the Nile, Egypt would be as barren as the rest of the Sahara; with it, Egyptian farms can produce two or even three crops every year. Egyptian civilization was nurtured by the Nile and protected from invasion by the sea to the north and the desert to the east and west.

White Nile and Blue Nile

Dramatic changes in the amount of water flowing through the Nile occur annually, reflecting the fact that two independent streams, the Blue Nile and the White Nile, join at Khartoum, the capital of Sudan (also called North Sudan). The White Nile issues from Lake Victoria, the second-largest freshwater lake on Earth. It subsequently tumbles down a series of falls and rapids to Lake Kioga and Lake Albert before spilling out into a huge swamp known as the Sudd in South Sudan. The Sudd was the great barrier to European explorers seeking the source of the White Nile; the vast swamps caused explorers traveling in boats up the Nile to lose their way, forcing them to turn back or perish, often having contracted malaria or some other endemic disease carried by insects of the swamps. As a result, the source of the White Nile was discovered in 1862 by John Hanning Speke, who traveled west from Zanzibar to Lake Victoria instead of following the upstream course of the Nile River.

In the Sudd, the White Nile—there known as the Bahr el-Jebel—is joined by several tributaries. From the west flows the Kiir River, also called Bahr al-Arab, and the Sobat River flows in from the east. The vastness of the Sudd results in huge losses due to evaporation, amounting to about 50 percent of the water flowing into it. The Sudd also acts as a buffer so that greater water flow into the Sudd causes the swampy area to expand, which in turn results in increased evaporation. Efforts to plan for expected increases in water demand, especially in Egypt, have focused on reducing these losses by building a 218-mile (350-kilometer) diversion, the Jonglei Canal, around the Sudd. The project was conceived in the 1940s, though construction did not begin until 1978. However, the resurgence of the Sudanese Civil War in 1983 halted the project, and growing environmental concerns made it unlikely that the canal would be finished quickly, even if the war ended. The present state of separation of South Sudan from North Sudan to form two independent nations has made the completion of the Jonglei Canal unlikely. The combination of relatively constant rainfall over the Lake Victoria region, the vastness of Lake Victoria, and evaporative losses in the Sudd result in a relatively constant flow of water down the White Nile to Khartoum. North of Malakal, the Nile flows over a flat stretch in a well-defined channel without swamps; the river drops only eight meters in elevation over the approximately 500 miles (800 kilometers) it covers from Malakal to Khartoum.

In contrast to the White Nile, the Blue Nile and its little sister, the Atbara River (Nahr ‘Atbarah), are seasonally affected streams. For about half of the year, they flow as feeble trickles, contributing negligible water to the Nile from January to June. In mid-to late summer, monsoons sweep large amounts of moisture evaporated from the Indian Ocean toward the Horn of Africa. As the water-laden air flows up over the Ethiopian highlands, it cools, and torrential rains fall. These rains swell the Blue Nile so much that its waters overwhelm the smaller White Nile at Khartoum and cause it to flow back upstream. Between 70 and 80 percent of the Nile’s total annual water budget results from the flood phase of the Blue Nile and the Atbara. These raging rivers also erode large amounts of black sand and silt from the basaltic highlands of Ethiopia and carry these sediments north. In his 1899 book The River War, Winston Churchill described the Nile in flood: “As the Nile rises its complexion is changed. The clear blue river becomes thick and red, laden with the magic mud that can raise cities from the desert sand and make the wilderness a garden.” The historic fertility of the Nile delta and valley in Egypt owe as much to the new layer of this rich soil added to the inundated fields as to the deep soaking. This annual flooding and delivery of sediment no longer occurs downstream of the Aswan High Dam, which was constructed in the 1960s to ensure that the water needs of Egypt could be met regardless of drought in the Nile headwaters.

Like the White Nile, the Blue Nile issues from a large lake, Lake Tana. It initially flows to the southeast but progressively turns to the southwest and then northwest as it descends through the Blue Nile canyon. It leaves the mountains near the border between South Sudan and Ethiopia and continues on a northwestward track across the Sudanese lowlands until it joins the White Nile at Khartoum.

Cataract Nile, Great Bend, and Egyptian Nile

For a distance of 1,148 miles (1,847 kilometers) from Khartoum to Aswan, the Nile is defined by two features: the cataracts and the great bend. The cataracts are sections where the river tumbles over rocky outcroppings, creating serious obstacles to navigation. There are six “classical” cataracts, but in reality, there are many more; as much as 350 miles (565 kilometers) of this portion of the Nile, referred to as the Cataract Nile, is affected by cataracts. The cataracts are also significant because they define river segments where granites and other resistant rocks come down to the edge of the Nile. The floodplain in this region is narrow to nonexistent, and opportunities for agricultural development are correspondingly limited. These two factors—the navigational obstacles and the restricted floodplain—are the chief reasons why the land around this part of the Nile is thinly populated and why the historic border between Egypt in the north and Nubia or Sudan in the south was found not far from the First Cataract at Aswan.

The great bend is one of the most unexpected features of the Nile. For most of its course, the Nile flows inexorably to the north. At one location in the heart of the Sahara, however, it turns southwest and flows away from the sea for 186 miles (300 kilometers) before resuming its northward journey. This deflection of the river’s course is the result of the tectonic uplift that created the Nubian Swell over the past few hundred thousand years. This uplift is also responsible for the cataracts. If not for this geologically recent uplift, these rocky stretches would have been eroded away long ago by the abrasive action of the sediment-laden Nile.

The northernmost segment is the Egyptian Nile, extending for approximately 745 miles (1,200 kilometers) between Aswan and the Mediterranean Sea. It consists of two parts, the Nile River valley and the delta. The Nile valley consists of the broad floodplain that is imprisoned between steep limestone or sandstone hillsides. The boundary between the lush valley floor and the flanking desert is stark and sudden; a visitor can stand with one foot on the black mud, brought three thousand kilometers from Ethiopia, and the other foot on desert sand or barren limestone. The floodplain widens progressively to the north until it opens up just north of Cairo into the delta. The mouth of the Nile is where the term “delta” was first applied; the ancient Greeks noted the triangular shape of the land around the Nile’s mouth and likened it to the shape of the Greek letter delta (Δ). The Nile splits into two branches at the south end of the delta: the western Rosetta branch, where the famous Rosetta Stone was discovered, and the eastern Damietta branch. Up to the time that the Aswan High Dam was built, the delta continued to grow as annual floods laid down their loads of silt. These sediments are now deposited in Lake Nasser, an artificial lake created by the dam, with the result that the sediment-starved delta is slowly sinking and its shoreline is retreating due to erosion by the waters of the Mediterranean Sea.

Geological Formation of the Nile

The evolution of the Nile is an important area of scientific research. The river consists of a series of steeper and flatter segments, which are thought to indicate that several independent drainage systems previously existed in the region now drained by the Nile. Much is known about when and how the course of the Nile came into being, but much remains to be learned.

A critical event in the formation of the Nile was the evaporation of the Mediterranean Sea about six million years ago. Because of the climatic zone in which it lies, more water evaporates from the Mediterranean than is supplied by the rivers that flow into it. This water deficit requires that replenishing seawater flow into the Mediterranean from the Atlantic. When tectonic movement closed the Strait of Gibraltar as the African and European continental masses collided, the influx of Atlantic Ocean water was stopped, and the Mediterranean slowly dried up. The Dead Sea of Israel and Jordan, at about 1,312 feet (400 meters) below sea level, is presently the deepest spot on the continents, but the Mediterranean sea floor, lying as much as 9,843 feet (3,000 meters) below sea level, was 1,969 feet (600 meters) deeper and one thousand times more vast than the Dead Sea region. This may have been the greatest sea ever to evaporate completely, and the event profoundly affected the streams that flowed into it. A north-flowing river existed in what is now Egypt; as the sea level dropped, the stream became steeper and steeper as it cut down into relatively soft limestones. The enhanced erosive power allowed its upper tributaries to extend into the headwaters and capture upstream drainages. The increased water from the captured streams further increased the stream’s erosive power, further stimulating the expansion of the drainage system upstream. This led to the development of the “Eonile” (the “dawn Nile”), which flowed through a huge canyon that was deeper than the Grand Canyon of Arizona and many times longer. This canyon is buried beneath the silt beds of the Egyptian Nile that have accumulated over the intervening millions of years, but it cannot be traced south of Aswan.

In time, the barrier at Gibraltar ruptured, and a tremendous waterfall brought Atlantic seawater to refill the Mediterranean basin. The “Grand Canyon of Egypt” became a drowned river valley or estuary, similar to the fjords of Norway but very different in origin. Slowly, this estuary filled with sediments brought in by rivers flowing from the south, and a landscape not too different from the present was established three million years ago.

Study of the Nile

It is much more difficult to discern the development of the Nile upstream from Aswan, but there are some clues. Lake Victoria did not exist prior to about twelve thousand years ago. Before this time, the streams of the Ugandan highlands flowed west to join the Congo, which drains into the equatorial Atlantic. Recent tectonic activity lifted and tilted the region, forming the lake and directing its overflow to the north. Similarly, recent tectonic activity changed the effects of the Nubian Swell and the Bayuda Uplift, with the result that northward flow across Nubia is sometimes permitted and sometimes blocked. Probably the oldest parts of the Nile drainage system are those associated with the Sudd. These follow the axes of sediment-filled rifts that formed more than sixty-five million years ago and that have continued to sink and fill with sediments since that time.

The Ethiopian highlands began to form about thirty million years ago as a result of tremendous volcanic activity, as a mantle plume punctured the crust. However, the contribution of the distinctive black sediments from these highlands is not recognized in the Egyptian Nile until about six-hundred and fifty thousand years ago. This may be the result of increased rainfall on the Ethiopian highlands accompanying the development and intensification of monsoonal circulation in the recent past. Monsoonal circulation is caused by the change in position of atmospheric low-pressure cells, which lie over the equatorial Indian Ocean during northern winter and over south-central Asia during northern summer. The result is that cold, dry winds blow southward from Asia during winter, but warm, moist winds blow northward from the sea toward Asia in summer. The westward deflection of summer winds resulting from the Coriolis effect brings part of the moisture-laden air currents over Ethiopia, where the air cools as it rises. Cool air can hold less moisture than warm air, so clouds and then rain form as the monsoon rises over the Ethiopian highlands. This brings the long, drenching rains in Ethiopia that cause the annual Nile flood. The monsoonal circulation has intensified over the last few million years as a result of the continued uplift of the Tibetan Plateau. The mystery of the Nile flood that puzzled the ancient Egyptians can thus best be understood by knowing about mountain-building events occurring thousands of kilometers away. Water from the Ethiopian highlands may not have reached Egypt because the Nubian Swell acted as a barrier, perhaps deflecting the water to the west. It may be that only with the additional water provided as a result of the intensifying monsoon was the upstream Nile able to erode its way through the Nubian Swell and continue north to the Mediterranean Sea.

The geological history of the Nile is mostly inferred from sedimentary deposits in the delta and the Egyptian Nile. Scientists know that great river systems carried sediments—preserved today as Nubian sandstone—north from central Africa as long ago as the Cretaceous period, about one hundred million years ago, but the course of these rivers is poorly known. No link can be established between the rivers of the Cretaceous period and the present-day Nile. The sea invaded Africa from the north toward the end of the Cretaceous period, and a large portion of northeast Africa was covered by a shallow sea during much of the early Tertiary period, about seventy million to forty million years ago. River deposits from the late Eocene and early Oligocene (about thirty-five million years ago) are known to the west of the present Nile, but these sediments did not travel far, indicating that they came from a relatively small river. This may have been the precursor of the stream that carved the great canyon following the evaporation of the Mediterranean Sea about six million years ago.

Scientists continued to study the Nile River in the twenty-first century. In late 2022, a study was released that revealed that when ancient Egyptians were erecting the famous Pyramids of Giza, the Nile River was in close enough proximity to the site to aid in the transport of construction materials. Scientists were able to determine the existence of this extinct branchknown as the Ancient Ahramat Branchof the Nile River through pollen analysis and with the help of radar satellites. This discovery has answered many questions as to why the pyramids were built where they were and how the ancient Egyptians were able to transport materials in order to construct the pyramids.

Principal Terms

cataract: rough water or a waterfall in a river, generally obstructing navigation

hydrology: the science that deals with the properties, distribution, and circulation of water on land

mantle plume: a rising current of extra-hot magma in the mantle

monsoon: a seasonal air current system; in the Northern Hemisphere’s winter, dry winds flow from the continents to the ocean, whereas in summer, moist winds flow from the ocean to the continents and cause heavy rains

pyroxene: a rock-forming mineral commonly found in igneous rocks such as basalt or gabbro

rift: a portion of the earth’s crust where tension has caused faulting, producing an elongated basin; rifts fill with sediments and, sometimes, volcanic rocks

tectonic: relating to differential motions and deformation of the earth’s crust, usually associated with faulting and folding of rock layers

Bibliography

Adar, Korwa G., and Nicasius A. Check, editors. Cooperative Diplomacy, Regional Stability and National Interests: The Nile River and Riparian States. Africa Institute of South Africa, 2011.

Al-Atawy, Mohamed-Hatem. Nilopolitics: A Hydrological Regime, 1870–1990. Special issue of Cairo Papers in Social Science, vol. 19, no. 1, 1996.

Awulachew, Seleshe Bekele, et al., editors. The Nile River Basin: Water, Agriculture, Governance and Livelihoods. Routledge, 2012.

Churchill, Winston. The River War: An Account of the Reconquest of the Sudan. 1899. Dover Publications, 2006.

Collins, Robert O. The Nile. Yale UP, 2002.

Darby, Stephen, and David Sear, editors. River Restoration: Managing the Uncertainty in Restoring Physical Habitat. John Wiley & Sons, 2008.

Guadalupi, Gianni. The Nile: History, Adventure and Discovery. Translated by Neil Frazer Davenport, White Star Publishers, 2007.

Hillel, Daniel. Rivers of Eden: The Struggle for Water and the Quest for Peace in the Middle East. Oxford UP, 1994.

Howell, P. P., and J. A. Allan, editors. The Nile: Sharing a Scarce Resource; A Historical and Technical Review of Water Management and of Economic and Legal Issues. Cambridge UP, 1994.

Marie, Mustafa. “Study Reveals How Ancient Egyptians Benefited from the Nile River's Course.” Egypt Today, 21 Sept. 2022, www.egypttoday.com/Article/4/119349/Study-reveals-how-ancient-Egyptians-benefited-from-the-Nile-River. Accessed 15 Apr. 2023.

Melesse, Assefa M., editor. Nile River Basin: Hydrology, Climate and Water Use. Springer, 2011.

Melesse, Assefa M., et al., editors. Nile River Basin: Ecohydrological Challenges, Climate Change and Hydropolitics. Springer, 2014.

Shoup, John A. The Nile: An Encyclopedia of Geography, History, and Culture. ABC-CLIO, 2017.

Williams, Riis. “Lost Branch of the Nile May Solve Long-Standing Mystery of Egypt's Famed Pyramids.” Scientific American, 16 May 2024, www.scientificamerican.com/article/egypts-famed-pyramids-overlooked-a-long-lost-branch-of-the-nile/. Accessed 3 Aug. 2024.

Wohl, Ellen. A World of Rivers: Environmental Change on Ten of the World's Great Rivers. U of Chicago P, 2010.