The boundaries of Otago have changed over time. However, the Waitaki River, currently Otago's northern extent, has often been used as a natural border. Ngāi Tahu, the dominant Maori tribe in the region, currently has three rūnanga (sub-tribes) within Otago and their traditional extent is not limited to the region. Today Otago is divided into the Central Otago, Clutha, Queenstown-Lakes and Waitaki (partly in Canterbury) Districts, and the city of Dunedin, which has half the region's population. It excludes the Southland plains, Stewart Island and Fiordland, although the historical province of Otago and much older Murihiku region often included them.