New evidence suggests climate change led to the collapse of the ancient Indus Valley civilization of south Asia 4,000 years ago, U.S. researchers say. The Bronze Age civilization, spread over what are now Pakistan, northwest India, and eastern Afghanistan, was the largest but least known of the first great urban cultures that also included Egypt and Mesopotamia. Like their contemporaries, the Harappans, named for one of their largest cities, lived next to rivers that annually watered their lands and allowed an agricultural society to flourish. However, numerous remains of Harappan settlements have been found in what is today a vast desert region far from any flowing river. Researchers from the Woods Hole Oceanographic Institution in Massachusetts say archaeological evidence combined with state-of-the-art geoscience technologies suggests a decline in monsoon rains led to weakened river dynamics and played a critical role in the collapse of the Harappan culture. \"The Harappans were an enterprising people taking advantage of a window of opportunity -- a kind of \"Goldilocks civilization,\" Woods Hole researcher Liviu Giosan said. \"As monsoon drying subdued devastating floods, the land nearby the rivers -- still fed with water and rich silt -- was just right for agriculture. This lasted for almost 2,000 years, but continued aridification closed this favorable window in the end.\" The Indus civilization, which had been built on bumper crop surpluses along the Indus and the Ghaggar-Hakra rivers in the earlier wetter era, could not sustain itself as the rivers dwindled and dried up, researchers said.