Archaeologists define civilizations based on three key factors: the ability to support large urban populations through agriculture, specialized division of labor, and proximity to rivers which provide fertile land. The Indus Valley civilization flourished from 3500 BCE to 1700 BCE along the Indus River in modern-day Pakistan and India. Archaeologists have discovered over 1,500 sites of the Indus Valley civilization, including large planned cities like Mohenjo-Daro with standardized bricks, streets laid out in grids, and centralized drainage systems, indicating a high level of social organization. While we cannot read the Indus Valley's written language, archaeology has revealed aspects of their trade networks, religious practices, and eventual decline around 1700 BCE