It is not completely clear how the disappearance of the Harappan (also known as Indus River Valley) Civilization occurred. Some scholars would even emphasize that it did not disappear. They would say, instead, that various aspects of that civilization lived on in later civilizations. If we accept that the civilization did disappear, it was most likely due to a variety of causes.
One likely cause of the decline of the civilization was environmental degradation. Scholars believe that the people of the Indus Valley deforested their region excessively. This led to such problems as massive flooding and the loss of topsoil taken away by the floods. They also believe that the intensive irrigation practiced by this civilization led to salinization of the soil. Together, these problems made agriculture much less productive.
These floods are also believed to have caused other problems. They are believed to have inundated cities, wrecking the economy and making people much less confident about their civilization. This would have weakened the government of the civilization.
Another possible cause is geological. Scholars believe that tectonic activity in the region raised the coast. This caused the Indus to stop reaching the sea and to, instead, form inland lakes. This disrupted the trade that was important to the economy.
Finally, the Indus River is believed to have changed its course while another river, the Saravasti, dried up completely. For civilizations reliant on the rivers for water and for transport, this would have been devastating. All of these factors combined, scholars believe, the help bring about the end of the Harappan Civilization.