Around 1800 BCE, signs of a gradual decline began to emerge, and by around 1700 BCE, most of the cities were abandoned. In 1953, Sir Mortimer Wheeler proposed that the decline of the Indus Civilization was caused by the invasion of an Indo-European tribe from Central Asia called the "Aryans".
As evidence, he cited a group of 37 skeletons found in various parts of Mohenjo-Daro, and passages in the Vedas referring to battles and forts. However, scholars soon started to reject Wheeler's theory, since the skeletons belonged to a period after the city's abandonment and none were found near the citadel. Subsequent examinations of the skeletons by Kenneth Kennedy in 1994 showed that the marks on the skulls were caused by erosion, and not violent aggression.
Today, many scholars believe that the collapse of the Indus Civilization was caused by drought and a decline in trade with Egypt and Mesopotamia. It has also been suggested that immigration by new peoples, deforestation, floods, or changes in the course of the river may have contributed to the collapse of the IVC.
Previously, it was also believed that the decline of the Harappan civilization led to an interruption of urban life in the Indian subcontinent. However, the Indus Valley Civilization did not disappear suddenly, and many elements of the Indus Civilization can be found in later cultures.
Current archaeological data suggest that material culture classified as Late Harappan may have persisted until at least around 1000-900 BCE and was partially contemporaneous with the Painted Grey Ware culture.
Recent archaeological excavations indicate that the decline of Harappa drove people eastward. After 1900 BCE, the number of sites in India increased from 218 to 853. Excavations in the Gangetic plain show that urban settlement began around 1200 BCE, only a few centuries after the decline of Harappa and much earlier than previously expected.
A possible natural reason for the IVC's decline is connected with climate change that is also signaled for the neighboring areas of the Middle East: The Indus valley climate grew significantly cooler and drier from about 1800 BCE, linked to a general weakening of the monsoon at that time.
Alternatively, a crucial factor may have been the disappearance of substantial portions of the Ghaggar Hakra river system. A tectonic event may have diverted the system's sources toward the Ganges Plain, though there is complete uncertainty about the date of this event, as most settlements inside Ghaggar-Hakra river beds have not yet been dated. The actual reason for decline might be any combination of these factors.
New geological research is now being conducted by a group led by Peter Clift, from the University of Aberdeen, to investigate how the courses of rivers have changed in this region since 8000 years ago, to test whether climate or river reorganizations are responsible for the decline of the Harappan.
As a consequence of the Indus Civilization's collapse, regional cultures emerged, to varying degrees showing the influence of the Indus Civilization. In the formerly great city of Harappa, burials have been found that correspond to a regional culture called the Cemetery H culture. At the same time, the Ochre Colored Pottery culture expanded from Rajasthan into the Gangetic Plain. The Cemetery H culture has the earliest evidence for cremation, a practice dominant in Hinduism till today.