What are the Ganges and Nile why might their valleys have high population densities What is it based on?

The Ganges River, also called Ganga, is a river located in northern India that flows toward the border with Bangladesh. It is the longest river in India and flows for around 1,569 miles (2,525 km) from the Himalayan Mountains to the Bay of Bengal. The river has the second greatest water discharge in the world, and its basin is the most heavily populated in the world with over 400 million people living in it.

The Ganges River is extremely important to the people of India as most of the people living on its banks use it for daily needs such as bathing and fishing. It is also significant to Hindus, as they consider it their most sacred river.

The headwaters of the Ganges River begin high in the Himalayan Mountains where the Bhagirathi River flows out of the Gangotri Glacier in India's Uttarakhand state. The glacier sits at an elevation of 12,769 feet (3,892 m). The Ganges River proper begins farther downstream where the Bhagirathi and Alaknanda rivers join. As the Ganges flows out of the Himalayas, it creates a narrow, rugged canyon.

The Ganges River emerges from the Himalayas at the town of Rishikesh where it begins to flow onto the Indo-Gangetic Plain. This area, also called the North Indian River Plain, is a very large, relatively flat, fertile plain that makes up most of the northern and eastern parts of India as well as parts of Pakistan, Nepal, and Bangladesh. In addition to entering the Indo-Gangetic Plain in this area, part of the Ganges River is also diverted toward the Ganges Canal for irrigation in the Uttar Pradesh state.

As the Ganges River then flows farther downstream, it changes its direction several times and is joined by many other tributary rivers such as the Ramganga, Tamsa, and Gandaki Rivers, to name a few. There are also several cities and towns that the Ganges River passes through on its way downstream. Some of these include Chunar, Kolkata, Mirzapur, and Varanasi. Many Hindus visit the Ganges River in Varanasi as that city is considered the holiest of cities. As such, the city's culture is also closely tied into the river as it is the most sacred river in Hinduism.

Once the Ganges River flows out of India and into Bangladesh, its main branch is known as the Padma River. The Padma River is joined downstream by large rivers like the Jamuna and Meghna rivers. After joining the Meghna, it takes on that name before flowing into the Bay of Bengal. Before entering the Bay of Bengal however, the river creates the world's largest delta, Ganges Delta. This region is a highly fertile sediment-laden area that covers 23,000 square miles (59,000 sq km).

It should be noted that the course of the Ganges River described in the above paragraphs is a general description of the river's route from its source where the Bhagirathi and Alaknanda rivers join to its outlet at the Bay of Bengal. The Ganges has very complicated hydrology, and there are several different descriptions of its overall length and the size of its drainage basin based on what tributary rivers are included. The most widely accepted length of the Ganges River is 1,569 miles (2,525 km), and its drainage basin is estimated to be about 416,990 square miles (1,080,000 sq km).

The Ganges River basin has been inhabited by humans since ancient times. The first people in the region were of the Harappan civilization. They moved into the Ganges River basin from the Indus River basin around the 2nd millennium BCE. Later, the Gangetic Plain became the center of the Maurya Empire and then the Mughal Empire. The first European to discuss the Ganges River was Megasthenes in his work Indica.

In modern times, the Ganges River has become a source of life for the nearly 400 million people living in its basin. They rely on the river for their daily needs such as drinking water supplies and food and for irrigation and manufacturing. Today, the Ganges River basin is the most populated river basin in the world. It has a population density of about 1,000 people per square mile (390 per sq km).

Aside from providing drinking water and irrigating fields, the Ganges River is extremely important to India's Hindu population for religious reasons as well. The Ganges River is considered their most sacred river, and it is worshiped as the goddess Ganga Ma or "Mother Ganges." 

According to the Myth of the Ganges, the goddess Ganga descended from heaven to dwell in the waters of the Ganges River to protect, purify and bring to heaven those who touch it. Devout Hindus visit the river daily to offer flowers and food to Ganga. They also drink the water and bathe in the river to cleanse and purify their sins.

Hindus believe that upon death the waters of the Ganges River are needed to reach the World of the Ancestors, Pitriloka. As a result, Hindus bring their dead to the river for cremation along its banks and afterward their ashes are spread in the river. In some cases, corpses are also thrown into the river. The city of Varanasi is the holiest of cities along the Ganges River and many Hindus travel there to place ashes of their dead in the river.

Along with daily baths in the Ganges River and offerings to the goddess Ganga, there are large religious festivals that occur in the river throughout the year where millions of people travel to the river to bathe so that they can be purified of their sins.

Despite the religious significance and daily importance of the Ganges River for the people of India, it is one of the most polluted rivers in the world. Pollution of the Ganges is caused by both human and industrial waste due to India's rapid growth as well as religious events. India currently has a population of over 1 billion people, and 400 million of them live in the Ganges River basin. As a result, much of their waste, including raw sewage, is dumped into the river. Also, many people bathe and use the river to clean their laundry. Fecal coliform bacteria levels near Varanasi are at least 3,000 times higher than what is established by the World Health Organization as safe (Hammer, 2007).

Industrial practices in India also have little regulation and as the population grows these industries do as well. There are many tanneries, chemical plants, textile mills, distilleries and slaughterhouses along the river and many of them dump their untreated and often toxic waste into the river. The water of the Ganges has been tested to contain high levels of things like chromium sulfate, arsenic, cadmium, mercury and sulfuric acid (Hammer, 2007).

In addition to human and industrial waste, some religious activities also increase the pollution of the Ganges. For example, Hindus believe that they must take offerings of food and other items to Ganga and as a result, these items are thrown into the river on a regular basis and more so during religious events. Human remains are also often placed into the river.

In the late 1980s, India's prime minister, Rajiv Gandhi began the Ganga Action Plan (GAP) to clean up the Ganges River. The plan shut down many highly polluting industrial plants along the river and allotted funding for the construction of wastewater treatment facilities, but its efforts have fallen short as the plants are not large enough to handle the waste coming from such a large population (Hammer, 2007). Many of the polluting industrial plants are also continuing to dump their hazardous waste into the river.

Despite this pollution, however, the Ganges River remains important to the Indian people as well as different species of plants and animals such as the Ganges River dolphin, a very rare species of freshwater dolphin that is native only to that area. To learn more about the Ganges River, read "A Prayer for the Ganges" from Smithsonian.com.

Asia is the largest of the world’s continents, covering approximately 30 percent of the Earth’s land area. It is also the world’s most populous continent, with roughly 60 percent of the total population.

Asia makes up the eastern portion of the Eurasian supercontinent; Europe occupies the western portion. The border between the two continents is debated. However, most geographers define Asia’s western border as an indirect line that follows the Ural Mountains, the Caucasus Mountains, and the Caspian and Black Seas. Asia is bordered by the Arctic, Pacific, and Indian Oceans.

Asia’s physical geography, environment and resources, and human geography can be considered separately.

Asia can be divided into five major physical regions: mountain systems; plateaus; plains, steppes, and deserts; freshwater environments; and saltwater environments.

Mountain Systems

The Himalaya mountains extend for about 2,500 kilometers (1,550 miles), separating the Indian subcontinent from the rest of Asia. The Indian subcontinent, once connected to Africa, collided with the Eurasian continent about 50 million to 55 million years ago, forming the Himalayas. The Indian subcontinent is still crashing northward into Asia, and the Himalayas are growing about 5 centimeters (2 inches) every year.

The Himalayas cover more than 612,000 square kilometers (236,000 square miles), passing through the northern states of India and making up most of the terrain of Nepal and Bhutan. The Himalayas are so vast that they are composed of three different mountain belts. The northernmost belt, known as the Great Himalayas, has the highest average elevation at 6,096 meters (20,000 feet). The belt contains nine of the highest peaks in the world, which all reach more than 7,925 meters (26,000 feet) tall. This belt includes the highest mountain summit in the world, Mount Everest, which stands at 8,849 meters (29,032 feet).

The Tien Shan mountain system stretches for about 2,400 kilometers (1,500 miles), straddling the border between Kyrgyzstan and China. The name Tien Shan means “Celestial Mountains” in Chinese. The two highest peaks in the Tien Shan are Victory Peak, which stands at 7,439 meters (24,406 feet), and Khan Tängiri Peak, which stands at 6,995 meters (22,949 feet). Tien Shan also has more than 10,100 square kilometers (3,900 square miles) of glaciers. The largest glacier is Engil'chek Glacier, which is about 60 kilometers (37 miles) long.

The Ural Mountains run for approximately 2,500 kilometers (1,550 miles) in an indirect north-south line from Russia to Kazakhstan. The Ural Mountains are some of the world’s oldest, at 250 million to 300 million years old. Millions of years of erosion have lowered the mountains significantly, and today their average elevation is between 914 and 1,220 meters (3,000 to 4,000 feet). The highest peak is Mount Narodnaya at 1,895 meters (6,217 feet).

Plateaus

Asia is home to many plateaus, areas of relatively level high ground. The Iranian plateau covers more than 3.6 million square kilometers (1.4 million square miles), encompassing most of Iran, Afghanistan, and Pakistan. The plateau is not uniformly flat, but contains some high mountains and low river basins. The highest mountain peak is Damavand, at 5,610 meters (18,410 feet). The plateau also has two large deserts, the Dasht-e Kavir and Dasht-e Lut.

The Deccan Plateau makes up most of the southern part of India. The plateau’s average elevation is about 600 meters (2,000 feet). It is bordered by three mountain ranges: the Satpura Range in the north, and the Eastern and Western Ghats on either side. The plateau and its main waterways—the Godavari and Krishna rivers—gently slope toward the Eastern Ghats and the Bay of Bengal.

The Tibetan Plateau is usually considered the largest and highest area ever to exist in the history of Earth. Known as the “Rooftop of the World,” the plateau covers an area about half the size of the contiguous United States and averages more than 5,000 meters (16,400 feet) above sea level. The Tibetan Plateau is extremely important to the world’s water cycle because of its tremendous number of glaciers. These glaciers contain the largest volume of ice outside the poles. The ice and snow from these glaciers feed Asia’s largest rivers. Approximately 2 billion people depend on the rivers fed by the plateau’s glaciers.

Plains, Steppes, and Deserts

The West Siberian Plain, located in central Russia, is considered one of the world’s largest areas of continuous flatland. It extends from north to south about 2,400 kilometers (1,500 miles) and from west to east about 1,900 kilometers (1,200 miles). With more than 50 percent of its area at less than 100 meters (330 feet) above sea level, the plain contains some of the world’s largest swamps and flood plains.

Central Asia is dominated by a steppe landscape, a large area of flat, unforested grassland. Mongolia can be divided into different steppe zones: the mountain forest steppe, the arid steppe, and the desert steppe. These zones transition from the country’s mountainous region in the north to the Gobi Desert on the southern border with China.

Freshwater

Lake Baikal, located in southern Russia, is the deepest lake in the world, reaching a depth of 1,620 meters (5,315 feet). The lake contains 20 percent of the world’s unfrozen freshwater, making it the largest reservoir on Earth. It is also the world’s oldest lake, at 25 million years old.

The Yangtze is the longest river in Asia and the third longest in the world (behind the Amazon of South America and the Nile of Africa). Reaching 6,300 kilometers (3,915 miles) in length, the Yangtze moves east from the glaciers of the Tibetan Plateau to the river’s mouth on the East China Sea. The Yangtze is considered the lifeblood of China. It drains one-fifth of the country’s land area, is home to one-third of its population, and contributes greatly to China’s economy.

The Tigris and Euphrates Rivers begin in the highlands of eastern Turkey and flow through Syria and Iraq, joining in the city of Qurna, Iraq, before emptying into the Persian Gulf. The land between the two rivers, known as Mesopotamia, was the center of the earliest civilizations, including Sumer and the Akkadian Empire. Today, the Tigris-Euphrates river system is under threat from increased agricultural and industrial use. These pressures have caused desertification and increased salts in the soil, severely damaging local watershed habitats.

Saltwater

The Persian Gulf has an area of more than 234,000 square kilometers (90,000 square miles). It borders Iran, Oman, United Arab Emirates, Saudi Arabia, Qatar, Bahrain, Kuwait, and Iraq. The gulf is subject to high rates of evaporation, making it shallow and extremely salty. The seabed beneath the Persian Gulf contains an estimated 50 percent of the world’s oil reserves. The countries that border the gulf have engaged in a number of disputes over this rich resource.

The Sea of Okhotsk covers 1.5 million square kilometers (611,000 square miles) between the Russian mainland and the Kamchatka Peninsula. The sea is largely frozen between October and March. Large ice floes make winter navigation almost impossible.

The Bay of Bengal is the largest bay in the world, covering almost 2.2 million square kilometers (839,000 square miles) and bordering Bangladesh, India, Sri Lanka, and Burma. Many large rivers, including the Ganges and Brahmaputra, empty into the bay. The briny wetlands formed by the Ganges-Brahmaputra on the Bay of Bengal is the largest delta in the world.

Terrestrial Flora and Fauna

Botanists nickname China the “Mother of Gardens.” It has more flowering plant species than North and South America combined. Because China has such diverse landscapes, from the arid Gobi Desert to the tropical rain forests of Yunnan Province, many flowers can adapt to climates all over the world. From roses to peonies, many familiar flowers most likely originated in northern China. China is the likely origin of such fruit trees as peaches and oranges. China is also home to the dawn redwood, the only redwood tree found outside North America.

Asia’s diverse physical and cultural landscape has dictated the way animals have been domesticated. In the Himalayas, communities use yaks as beasts of burden. Yaks are large animals related to cattle, but with a thick fiber coat and the ability to survive in the oxygen-poor high altitude of the mountains. Yaks are not only used for transportation and for pulling plows, but their coats are sources of warm, hardy fiber. Yak milk is used for butter and cheese.

In the Mongolian steppe, the two-humped Bactrian camel is the traditional beast of burden. Bactrian camels are critically endangered in the wild. The camel’s humps store nutrient-rich fat, which the animal can use in times of drought, heat, or frost. Its size and ability to adapt to hardship make it an ideal pack animal. Bactrians can actually outrun horses over long distances. These camels were the traditional animals used in caravans on the Silk Road, the legendary trade route linking eastern Asia with India and the Middle East.

Aquatic Flora and Fauna

The freshwater and marine habitats of Asia offer incredible biodiversity.

Lake Baikal’s age and isolation make it a unique biological site. Aquatic life has been able to evolve for millions of years relatively undisturbed, producing a rich variety of flora and fauna. The lake is known as the “Galápagos of Russia” because of its importance to the study of evolutionary science. It has 1,340 species of animals and 570 species of plants.

Hundreds of Lake Baikal’s species are endemic, meaning they are found nowhere else on Earth. The Baikal seal, for instance, is one of the few freshwater seal species in the world. The Baikal seal feeds primarily on the Baikal oil fish and the omul. Both fishes are similar to salmon, and provide fisheries for the communities on the lake.

The Bay of Bengal, on the Indian Ocean, is one of the world’s largest tropical marine ecosystems. The bay is home to dozens of marine mammals, including the bottlenose dolphin, spinner dolphin, spotted dolphin, and Bryde’s whale. The bay also supports healthy tuna, jack, and marlin fisheries.

Some of the bay’s most diverse array of organisms exist along its coasts and wetlands. Many wildlife reserves in and around the bay aim to protect its biological diversity.

The Sundarbans is a wetland area that forms at the delta of the Ganges and Brahamaputra rivers. The Sundarbans is a huge mangrove forest. Mangroves are hardy trees that are able to withstand the powerful, salty tides of the Bay of Bengal as well as the freshwater flows from the Ganges and Brahamaputra. In addition to mangroves, the Sundarbans is forested by palm trees and swamp grasses.

The swampy jungle of the Sundarbans supports a rich animal community. Hundreds of species of fish, shrimp, crabs, and snails live in the exposed root system of the mangrove trees. The Sundarbans supports more than 200 species of aquatic and wading birds. These small animals are part of a food web that includes wild boar, macaque monkeys, monitor lizards, and a healthy population of Bengal tigers.