States that flourished in the area immediately south of the Zambesi River from the 15th to the 19th centuries were ruled by Karanga dynasties and were the cultural heirs of Great Zimbabwe. The most important of these states was Mokaranga, whose rulers bore the title of Monomotapa. Other important states—Teve, Manica, Barue, and Butua—all depended on the mining and trading of gold. Commerce was conducted at fairs attended by merchants from coastal towns such as Sofala and Chibuene, which were part of the networks of Indian Ocean commerce. At the beginning of the 16th century this trade attracted Portuguese traders who visited the fairs. In the 17th century, the Portuguese gradually expanded their presence through the institution of the prazos, whose owners acquired jurisdiction over extensive areas formerly ruled by the Karanga. The Portuguese were expelled from the Zimbabwe plateau in the 1690s and were succeeded by the Rosvi, another Karanga ruling elite. These states were devastated by droughts from the 1790s to the 1830s. All of them experienced civil wars before they were conquered by the Ngoni, who established the kingdom of Gaza, which covered the whole area south of the Zambesi as far as the Limpopo River until the time of the Scramble for Africa. Some of the old Karanga states, notably Manica and Barue, survived as tributaries of the Gaza state.