Around 1800 BC, signs of a gradual decline began to emerge, and by around 1700 BC most of the cities had been abandoned. Suggested contributory causes for the localisation of the IVC include changes in the course of the river, and climate change that is also signalled for the neighbouring areas of the Middle East. many scholars believe that drought led to a decline in trade with Egypt and Mesopotamia contributing to 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, linked to a general weakening of the monsoon at that time. The Indian monsoon declined and aridity increased, with the Ghaggar-Hakra retracting its reach towards the foothills of the Himalaya, leading to erratic and less extensive floods that made inundation agriculture less sustainable. Aridification reduced the water supply enough to cause the civilisation's demise, and to scatter its population eastward. As the monsoons kept shifting south, the floods grew too erratic for sustainable agricultural activities. The residents then migrated towards the Ganges basin in the east, where they established smaller villages and isolated farms. However trade with the old cities did not flourish. The small surplus produced in these small communities did not allow development of trade, and the cities died out. The Indo-Aryan peoples migrated into the Indus River Valley during this period and began the Vedic age of India. The Indus Valley Civilisation did not disappear suddenly and many elements of the civilization continued in later Indian subcontinent and Vedic cultures.