The greater Indus region was home to the largest of the four ancient urban civilizations of Egypt, Mesopotamia, South Asia and China. The Indus Valley Civilization was established around 3300 BC. It flourished between 2600 BC and 1900 BC. It started declining around 1900 BC and disappeared around 1400 BC. Harappa was a fortified city  believed to have been home to as many as 23,500 residents living in sculpted houses with flat roofs made of red sand and clay. The city spread over 150 hectares (370 acres) and had fortified administrative and religious centres.  There may have been another large river that ran parallel to and east of the Indus at some time in the past. The northern part of its now usually dry bed is called “Ghaggar” in India and “Hakra” in Pakistan. This group of channels is sometimes referred to as “Saraswati” and is associated by some with the Saraswati River of the Rg Veda).Its lost banks are slowly dried.