The Great Ruaha River is a river in south-central Tanzania that flows through the Usangu wetlands and the Ruaha National Park east into the Rufiji River. The population of the basin is mainly sustained by irrigation and water-related livelihoods such as fishing and livestock keeping. Great Ruaha is about 475 km (300 mi.) long, its tributary basin has a catchment area of 68,000 km² and the mean annual discharge is 140 m³/s. The Great Ruaha River supplies 22 per cent of the total flow of the Rufiji catchment system. Thirty-eight species of fish have been identified in the Great Ruaha River.