Tropical forest and the Ubangi and Uele rivers, major tributaries of the Congo River, dominate the landscape of this region. Other than the centralized Zande and Mangbetu, peoples traditionally were dispersed and politically decentralized. The Mangbetu established a major empire in the 18th century and are believed to have influenced neighboring peoples. Most Zande sculpture tends to be minimal and schematic. Among the few exceptions are figurative harps.