Explanation: Many Indus cities had been abandoned by 1900 BC. The Indus civilization collapsed due to the changes of the geography and climate of the area.
The movement of the outer layer has caused the Indus river to flood and change its direction. The changes in the flow of river had brought a terrible effect on the main cities of Indus as they were connected with the river.
The drains became blocked as the overcrowded people stopped maintaining the drains. The Indus people could have shifted to a place with less scale floods.