A fertile area watered by snowmelt from mountains, the broad floodplain of the Indus allowed for extensive irrigation, which hastened the development of a well-organized society fed by agricultural surpluses.The Indus Valley was also close to the Middle East and profited from trade with Mesopotamia, a region in what is now Iraq.

By 2500 B.C. cities that rivaled the great urban centers of Mesopotamia were developing along the Indus River and its tributaries. The most important of these cities were Mohenjo Daro, on the lower Indus River, and Harappa, on the Ravi River near the upper end of the Indus Valley.

Both cities were built on a similar plan, with a gridwork of streets, standardized housing for the common people and larger residences for the elite, and a sanitation system that included bathrooms linked to sewers—an important contribution to public health in cities that contained as many as 40,000 people.

Such urban planning reflected a complex society whose leaders could command the efforts of thousands of laborers, guided by engineers and officials. Bricks used for construction were all of the same mold, and the public buildings included granaries filled with surpluses that fed the leadership, the bureaucracy, and the many artisans who produced trade items.

Trade was the glue of the Harappa civilization, binding one city to another and the region as a whole to Mesopotamia and other distant lands. Among the goods exported were cotton, spices, ivory, and handcrafts such as jewelry. Ships hugged the coast of the Arabian Sea, bearing Harappan merchants and their wares to Ur and other Sumerian cities. Some of those traders settled in Mesopotamia.

Like other river valleys that fostered ancient civilizations, the Indus region was subject to seasonal flooding that helped nourish the fields but was sometimes catastrophic. The city of Mohenjo Daro had to be rebuilt at least nine times. Ruinous floods may have contributed to the decline of Harappan civilization after 2000 B.C. The area remained populous and productive, but the cities were abandoned and long-distance trade withered.