by a ridge into two parallel valleys, with a tributary, the Lukusashi River in a 25 km-wide valley to the north-west, and the Luangwa in a 15 km wide valley to the southeast. The river meanders less, and the flood plain narrows. The principal settlement in the Middle and Upper Luangwa Valley is Mfuwe which serves the tourism industry and has an international airport. Very few humans otherwise inhabit the valley. Lower Luangwa At 600 km the river abruptly enters a narrow valley between hills rising some 200 m from the broader valley floor, becoming almost a gorge. About 700 km from source the Luangwa merges with its tributary the Lukusashi after the latter has merged with the Lunsemfwa River coming from the opposite direction, and turns due south through a steep narrow valley: this is its exit from the Luangwa Rift Valley (see following section). After only 20 km it emerges from the hills into the broad valley of the Zambezi and meanders over sandy flats about 1.5 km wide in a flood plain of 3–5 km wide. It merges with the deeper Zambezi at Luangwa town. The lower section of the river forms the border between Zambia and Mozambique. The Luangwa Rift Valley This section explains the geomorphology of the Luangwa Valley. It is a rift valley or graben forming a south-west extension of the east African Rift, branching off its Lake Rukwa-Lake Malawi southern section, and reaching almost as far as Lusaka. The junction is not obvious because it filled with material spewed out from an ancient, extinct volcano. There are at least 20 hot springs, characteristic of a rift valley, in the valley or on its escarpments. The Luangwa flows along four-fifths of the Luangwa Rift Valley to the point where it meets the Lukusashi and the Lunsemfwa which has come from the opposite direction. At one time, millions of years ago, there was no way out and the Luangwa Rift filled with a Rift Valley Lake called the Madumabisa Lake, which rivalled Lake Malawi in size. The water of the lake overflowed in a river to the south-west, towards what is now the Kalahari, where it combined with the Okavango, Upper Zambezi, Cuando and Kafue rivers, emptying into the Limpopo River and flowing to the Indian Ocean. Several geological events combined to produce the current river systems. Faulting produced another graben just to the south