Of the three ancient river civilizations, we know the least about Harappan society located in the Indus River vlley. What are the reasons scholars know the least about this society? Given how little information we have, what evidence compels us to include it, along with Mesopotamia, Egypt, and China as one of the four ancient complex societies. When did Harappan society decline? After the decline of Harappan society, Aryans migrated into the Indian subcontinent. What are the lasting cultural, social and religious impacts of the migration of Aryans into the Indus River valley?