The decline of Harappan culture is difficult to explain. During its late
phase between 2000 and 1700 BC 'The Indus Valley Civilization as a distinct
entity gradually ceased to exist'. Historians have different opinions
regarding the causes of the decay and disappearance of the Harappan culture.
Various causes have been ascribed for its weakening and then decay: Increase
in rainfall, earthquake, decrease in fertility of soil, floods, Aryan
invasion, disease etc.
Mortimer Wheeler pointed out that the Harappan culture was destroyed by the Aryans. The Aryans were more skilled at warfare and were powerful than the Harappans. In the last phase of Mohenjodaro, men and women and children were massacred in the streets and houses. But there is very little evidence on this opinion.
Sir John Marshal, Lambrick and E.J.H Mackay suggest that the decline of the
Harappan civilization was mainly due to the vagaries of the Indus River. But
this theory is partly true. Some of the evidence of the devastation by floods
has been found at Mohenjodaro and Lothal but there is no such evidence in
respect of other sites like Kalibangan.
Some historians suggest that the first urban civilization came to an end around 1700 BC because its numerous small settlements grew beyond their natural limits leading to the mismanagement of natural resources. Although the theory of ecological factors for the decline of the Harappan civilization is latest yet it does not give us complete answer. Historians are of the view that the decline of the Indus Civilization was not the result of a single event; it was a slow decline and a result of combination of factors.