The decline of Indus valley civilization is a controversial historical topic. Many Historians have given their view regarding the decline of civilization. Reasons include natural calamities, foreign invasion, ecological mismanagement and various others including earthquakes and diseases. Many of the Historian agree that harrapan culture did not decline at once, but it was a gradual process that took a few centuries.
Some of the major reasons involved for the decline of Indus Valley Civilization:
Historians like John Wheeler belive that around 1500 BC Aryans from Central Asia or Europe conquered Indus Valley. The Aryans were nomads who were having extraordinary warfare skills. Harrapans on contrary were weak as they always led a very settled and peaceful life around Indus River. The Aryans burnt their settlement and killed Harappans. This view is controversial as there are very less evidence to support this theory.
Historians like Marshal and Mackay have view point that around 1500 BC due to various climatic factors Indus river changed it’s course and there were widespread floods in the region. This may have lead to outbreak of various waterborne disease and death of huge population. Flood based evidence are found in Mohenjo-daro but not in Kaliabangan. So, this theory is also not completely acceptable.
The Harrapans started settling very fast beyond their boundaries. They may have uprooted many trees and hunted many animals and due to lack of forest and animals these people may have dispersed in different directions. They shifted to the Deccan Plateau Region or Gangetic Plains of India.