River valley civilization

A river valley civilization is an agricultural nation or civilization situated beside and drawing sustenance from a river. A river gives the inhabitants a reliable source of water for drinking and agriculture. Some other possible benefits for the inhabitants are fishing, fertile soil due to annual flooding, and ease of transportation.

Overview
Civilizations tended to grow up in river valleys for a number of reasons. The most obvious is access to a usually reliable source of water for agriculture and other needs. Plentiful water and the enrichment of the soil due to annual floods made it possible to grow excess crops beyond what was needed to sustain an agricultural village. This allowed some of the people to engage in non-agricultural activities, such as the construction of buildings and cities (the root of the word "civilization"), metalworking, trade, and social organization. Boats on the river provided an easy and efficient way to transport people and goods, allowing the development of trade and facilitating central control of outlying areas.

Some examples

 * The Uruk period of Mesopotamia dates from about 4000 to 3100 BC, and provides the earliest signs of the existence of states in the Near East. Located along the Tigris and Euphrates rivers in West Asia, the name later given to that civilization, Mesopotamia, means "between rivers".
 * The Nile valley in Egypt had been home to agricultural settlements as early as 5500 BCE, but the growth of Ancient Egypt as a civilization began around 3100 BCE.
 * A third civilization grew up along the Indus River around 3300 BCE in parts of what are now India and Pakistan (see Bronze Age India).
 * The fourth great river civilization emerged around 1700 BCE along the Yellow River in China.

Mesopotamia
Mesopotamia was one of the earliest river valley civilizations: it started to form around 4000 BCE. The civilization was created after regular trading relationships started between multiple cities and states around the Tigris and Euphrates Rivers. Mesopotamian cities became self-run civil governments. One of the cities within this civilization, Ur, was the first literate society in history. Eventually, they constructed irrigation systems to exploit the two rivers, transforming their dry land into an agriculturally productive area, allowing population growth throughout the cities and states within Mesopotamia.

Nile River
Ancient Egypt also created irrigation systems from its local river, the Nile River, more complex than previous systems. The Egyptians would rotate legumes with cereal, which would stop salt buildup from the freshwater and enhance the fertility of their fields. The Nile River also allowed easier travel, eventually resulting in the creation of two kingdoms in the north and south areas of the river until both were unified into one society by 3000 BCE.

Indus valley
Much of the history of the Indus valley civilization is unknown. Discovered in the 1920s, Harappan society was larger than either Egypt or Mesopotamia. Historians have found no evidence of violence or a ruling class; there are no distinctive burial sites, and there is not a lot of evidence to suggest a formal military. Because the Harappan system of writing has not yet been deciphered, we know little about the ruling class and the military.

Yellow River
The Yellow River was settled in 9500 BCE. Many tribes settled along the river, sixth-longest in the world, which was distinguished by its heavy load of yellow silt and its periodic devastating floods. A major impetus for the tribes to unite into a single kingdom by around 1700 BCE (Erlitou culture, a Yellow River civilization) was the desire to find a solution to the frequent deadly floods. The Yellow River is often called "The Cradle of Chinese Civilization".

Supe Valley
Caral-Supe (also known as Caral and Norte Chico) was a complex pre-Columbian-era society that included as many as thirty major population centers in what is now the Caral region of north-central coastal Peru. The Caral–Supe civilization flourished between the fourth and second millennia BC, with the formation of the first city generally dated to around 3500 BC, at Huaricanga, in the Fortaleza area. Large-scale human settlement and communal construction from 3100 BC onward is clearly apparent; it lasted until a period of decline around 1800 BC. Since the early 21st century, it has been established as the oldest-known civilization in the Americas, and as one of the six sites where civilisation separately originated in the ancient world.