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.[34] As of 2016[update], many scholars believed that drought and a decline in trade with Egypt and Mesopotamia caused the collapse of the Indus Civilisation.[35] The Ghaggar-Hakra system was rain-fed,[36][37][38] 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.[36] Aridity increased, with the Ghaggar-Hakra River retracting its reach towards the foothills of the Himalayas,[36][39][40] 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.[5][41][42][43]