To date whatever evidence, we have not been able to describe the reason for the decline of the Harrapan civilization. At the time around 2200 BCE, something didn’t work for the Harrapan people and it started to decline. They came to a total end by 2000 BCE and someplace maybe 1800 BCE.
Historians put forward some theories to explain the decline of the Harrapans.
1. Aryan Invasion
This one is the most famous reason for the Harrapan decline. They invaded Harrapan around this time. They had a fear fight and they put to an end this advanced civilization. However, Historians didn’t find any evidence to prove this theory. If there is an invasion, it should have some traces in archaeological records but there is no evidence of the military assault or conflict at any Harrapan site.
2. Flood
Another reason for their decline would be floods or natural disasters. Several layers of silt at Mohenjo-Daro give evidence that the city was affected by the repeated episodes of the Indus flood. (Historians assume that Mohenjo-Daro was the main center of the Harrapan civilization so after its decline many cities that were dependent on it also declined.) However, there was no such evidence found at any other Harrapan sites. So, this reason also does not completely explain the decline of the Harrapan people.
3. Gradual decline
Most Historians believe that Harrapan cities were not met a sudden end. But they gradually declined. Reasons for the gradual declines were
1. Drying up Rivers
Most of the Harrapan sites were located on the bank of the Ghaghra-Hakra river which was the ancient Sarasvati River. Around 2000 BCE due to climate change rain reduce in this area and River Saraswati dried up. Harrapan cities depending on this river had to relocate further east and south.
2. Environmental Change
The issue of environmental change can be connected to the ways in which Harrapans were treating the environment. Perhaps they are over-exploiting it through over-cultivation and excessive cutting of trees for fuel and farming. This could have resulted in decreasing soil fertility, floods, and increasing soil salinity.
3. Decline in Trade
The decline in Lapis Lazuli trade with Mesopotamia was a factor in the decline of the Harrapan civilization. Whether this trade is important or not to the Harrapan people is however debatable, constantly and could not consider the major factor for the decline.
Archeological evidence does not point to any direct causes for the decline. What it does show is the Harrapan cultural went through a gradual decline. The mature Harrapan phase is followed by the Late Harrapan cultural phase.
In the end, we can say that Harrapan culture was the first urban culture in south Asia. This civilization showed a varied subsistence base, vibrant craft tradition, and extensive trade networks. But we are still not able to decipher their text so there are still speculations about their religion, social life, and ruling policy. Right now, from the current studies, we can say that the Harrapan civilization did not come to a sudden end but gradually declined. Some historians also believe that there was never been any Aryan Invasion. But the Harrapan culture is inherited from the Vedic culture. There is still an investigation going on to prove this theory.