### Theory:

One of the most favourite topics in Indian archaeology has been the decline of the Harappan civilization. Several hypotheses have been proposed, ranging from environmental factors such as droughts, floods, shifting river courses, and the sea levels to tectonic changes caused by Aryan invasions, as well as trade disturbances and social instability. These explanations are re-examined below in light of new findings from recent explorations and excavations..
• They are Foreign Invasion.
• Environmental Changes
• Social Instability
The Foreign invasion:
Mortimer Wheeler was the first archaeologist to propose a hypothesis for the Harappan Civilisation's collapse.

He concluded that Aryan invasions devastated the Harappan Civilisation, based on circumstantial and historical evidence; for example, skeletons discovered buried in the streets of Mohenjo-Daro suggested that a war had occurred. To prove his hypothesis, he used the Rigveda as a significant source of historical data. According to the Rigveda, the conflict was a war between the incoming Aryans and the native Harappans.
Environmental Changes:
Some experts believed that the drying of the Saraswati river, which began around $$1900 BCE$$, was the leading cause for climate change.
Such significant environmental changes caused by a river's shifting course, such as deforestation, flooding, or drought.

Skeletal remains indicate that most people died due to malaria infection, which is infectious and spreads mainly through mosquitoes. This may have also resulted in a breakdown of the economy and public order in the cities.
The monsoon that flooded the Indus River Valley aided in agricultural growth, which helped create Harappan cities. The population began to depend on seasonal monsoons rather than irrigation, and the water supply would have dried up as the monsoons moved eastward.