Urban centers of the Indus Valley Civilisation were abandoned and replaced by disparate local cultures because of the same climate change that affected the neighbouring regions to the west. s of 2016[ [update]], many scholars believed that drought and a decline in trade with Egypt and Mesopotamia caused the collapse of the Indus Civilisation. The Ghaggar-Hakra system was rain-fed, and water supply depended on the monsoons. The Indus Valley climate grew significantly cooler and drier from about 1800 BC, which is linked to a contemporary general weakening of the monsoon. Aridity increased, with the Ghaggar-Hakra River retracting its reach towards the foothills of the Himalayas, leading to erratic and less-extensive floods, which made inundation agriculture less sustainable. Aridification reduced the water supply enough to cause the civilisation's demise, and to scatter its population eastward.