The Luangwa River is one of the major tributaries of the Zambezi River, and one of the four biggest rivers of Zambia. The river generally floods in the rainy season (December to March) and then falls considerably in the dry season. It is one of the biggest unaltered rivers in Southern Africa and the that make up the surrounding valley are home to abundant wildlife. Source and Upper-Middle Luangwa Valley Note: distances stated are approximate straight-line distances from source. The Luangwa rises in the Lilonda and Mafinga Hills in north-east Zambia at an elevation of around 1500 m, near the border with Tanzania and Malawi, and flows in a southwesterly direction through a broad valley. About 150 km from its source it has dropped to an elevation of about 690 m and becomes a meandering river with a flood-plain several kilometres wide. Over the next 300 km the meanders increase, with many oxbow lakes and abandoned meanders. Near Mfuwe, the river's elevation has dropped to about 520 m, the flood plain is about 10 km wide and the valley reaches about 100 km wide, with a north-west escarpment (Muchinga Escarpment) about 700 m high, and a south-western escarpment about 450 m high. In the dry season some sections, especially in the upper reaches, dry out completely, leaving isolated pools. The upper and middle parts of the valley contain the North Luangwa National Park and South Luangwa National Parks of Zambia, which are among some of the finest in Africa. The river itself is home to large populations of hippopotami and crocodiles. The world's largest concentration of hippos lives in the Luangwa Valley. In the dry season they are restricted by the shrinking river and pools, and are easily seen especially in isolated pools. In addition to being a source of water, the oxbow lakes and pools increase the biodiversity of the valley in other ways. The hippopotami which live in them feed on land vegetation at night. Their dung feeds some fish and fertilises the pools, increasing fish life which in turn feed crocodiles and birds. In the dry season, the grazing land animals and their predators congregate near the river and pools, and are easily seen. In the rainy season they graze further afield and are more easily hidden in the growth of new vegetation. At about 500 km the valley narrows to about 50 km and becomes divided