The decline of Harappan culture is difficult to explain. During its late phase between 2000 and 1700 BC 'The Indus Valley Civilization as a distinct entity gradually ceased to exist'. Scholars have given different answers to the question as to why did the civilization end? Some scholars, believing in a dramatic collapse of the civilization, have looked for evidences of a calamity of catastrophic proportions, which wiped out the urban communities. Some of the more plausible theories for the decline of the Harappan civilization are:
According to H. Weiss, from about 1800 B.C, the Indus Valley climate grew significantly cooler and drier. This is a main factor of the disappearance of the Ghaggar Haka river system, and leading to the decline of land of land and agriculture, therefore the Harappan had evacuated to other area. This is considered as the major reason for the decline of the Indus Valley. But on one has identified the basic dynamics of societal dissolution. Although this particular factor is not generally accepted, it will have been due to a combination of various reasons.
R.L Raikes views that Harappan civilization might be declined due to the disaster flooding and the earthquakes. The remains of Harappan civilization including the houses and streets were covered with slime and tumble building. It evidenced that this town ever had been the various layers of deep flooding. He also evidenced that some sites near Karachi were sea ports of Harappan civilization, due to it had been the earthquake, therefore, today it is situated far away from the sea coast. H.T Lambrick criticized this theory.
H.T. Lambick suggest that shifting away of the Indus River is the reason. He point out that the Indus River keeps on shifting its course about 35 miles away from Mohenjodaro, which of this area deserted it due to shortage of agricultural land and also starved of water. But this theory cannot explain the decline of other