Global Drought 2200-1900 BC
A joint French-American study of northern Mesopotamia just noted (Wiess, Coutry et. al. 1993) that the Akkadian empire of Mesopotamia, which had extensive trade links with the Harappans, was brought to an abrupt end as the result of a severe drought that began about 2200 BC and persisted for nearly 300 years. They attributed this to a global climatic change of increasing aridity that began nearly a thousand years earlier climaxing in what is now called the Great Drought of c. 2200-1900BC. Since the mature Vedic Harappan civilisation is also known to have ended around the same time the cause of the end is very obvious and common to all great civilisations of the period the Egyptian, Mesopotamian and the Harappan.
Ref: Sarasvati and the Vedic Civilisation by NS Rajaram