Rivers and Rains, Episode 12: When the Rains Moved
1Welcome back. I'm your host, Mateo Ruiz. Today we're comparing two great river plains of South Asia. In the west is the plain of the Indus River, mostly in what is now Pakistan. Far to the east is the plain of the Ganges River, in northern India and Bangladesh. Between much of the two lies a dry land of sand and scrub called the Thar Desert.
2Start with the Indus plain. It's dry. Most of its water comes from the Indus and its branches, which carry melting snow from the Himalayas and rain from the summer monsoon. A monsoon is a seasonal wind; between about June and September it blows moist air in from the ocean. On the Indus plain, people have long planted wheat and barley in the cool season, on land the summer flood has soaked.
3By about 2600 BCE, the people of the Indus Valley had built large, planned cities such as Harappa and Mohenjo-daro. Historians estimate that Mohenjo-daro may have held about 40,000 people. Builders laid out straight streets and covered drains, and they raised whole neighborhoods on brick platforms above the flood level.
4Then the climate changed. Mud from lake bottoms and rock layers in caves suggest that, after about 2200 BCE, the summer monsoon grew weaker for a long time. Some rivers in the eastern part of the Indus region seem to have dried up or changed course. In the centuries that followed, most of the big cities were left empty or shrank into small towns. At the same time, the number of small farming villages grew farther east, toward the Ganges plain, where more rain fell. Scientists still debate how much the climate caused these changes; problems with trade may also have played a part.
5The Ganges plain gets far more monsoon rain, and in ancient times forest covered much of it. Historians think that after about 1000 BCE, farmers with iron axes and plows cleared more and more of that forest and planted rice in flooded fields. By about 500 BCE, kingdoms with large cities had grown up along the river. Today the Ganges plain is one of the most crowded farming regions on Earth.