1. Causes of the decline of this civilization have not been firmly established. Archaeologists now believe that the civilization did not come to an abrupt end but gradually declined. People moved eastwards and cities were abandoned. Writing and trade declined. 2. Mortimer Wheeler suggested that the Aryan invasion led to the decline of the Indus Valley. This theory has now been debunked. 3. Robert Raikes suggests that tectonic movements and floods caused the decline. Other causes cited include a drying up of the rivers, deforestation, and destruction of the green cover. It is possible that some cities were destroyed by floods but not all. 4. It is now accepted that several factors could have led to the decline of the Indus Valley civilization.