Multiple choice What are the two main theories for why the Harappan civilization fell? Indo-Aryan Invasion Theory and Tsunami Theory Indo-Aryan Invasion Theory and Climate Change Theory Climate Change Theory and Mesopotamian War Theory None of the above Reveal answer Fill a bubble to check yourself B Correct answer Explanation The two prominent theories regarding the decline of the Harappan civilization are the Indo-Aryan migration/invasion theory and the theory that climate change, such as shifting river patterns or drought, made the region uninhabitable.