Decline of Indus Valley Civilization

How did the Indus Valley Civilization decline?  There is fierce controversy about it among experts. While one group finds an Aryan invasion as the cause, another group of investigators conclude that some internal factors such as salination of the soil, flooding, encroachment of the Rajasthan desert or even the change in the course of the Indus river – one of these or all together may have became the cause.  Another finding point to a tectonic activity, as a result of which the rivers went dry and their cities perished and that was the end of it.