Final answer:
The Indus River Civilization was marked by its advanced societal development with impressive urban planning, water management, and mathematical systems. Its decline was due to environmental factors, highlighting its unique structure potentially managed by religious leaders without centralized rule.
Step-by-step explanation:
The Indus River Civilization, also known as the Harappan Civilization, was one of the four early cradles of civilization, alongside Mesopotamia, Egypt, and China. Its notable characteristics include sophisticated urban planning, extensive water management systems, and a merchant class engaging in long-distance trade. In sites such as Mohenjo-Daro and Harappa, evidence shows advanced society with indoor plumbing, grid-like street layouts, and complex weight and measurement systems, suggesting a high level of mathematical understanding. Unique elements to the Indus Valley culture suggest an innovative and self-sustaining development apart from Mesopotamian influence. Its downfall around 1700 BCE is attributed to environmental changes rather than military conquest, showing a complex societal structure possibly managed by priest-like figures without a centralized state.