Lake Tanganyika - Geography

Geography

The lake is situated within the Albertine Rift, the western branch of the geographic feature known as the East African Rift, and is confined by the mountainous walls of the valley. It is the largest rift lake in Africa and the second largest lake by volume in the world. It is the deepest lake in Africa and holds the greatest volume of fresh water. It extends for 676 km (420 mi) in a general north-south direction and averages 50 km (31 mi) in width. The lake covers 32,900 km2 (12,700 sq mi), with a shoreline of 1,828 km (1,136 mi) and a mean depth of 570 m (1,870 ft) and a maximum depth of 1,470 m (4,820 ft) (in the northern basin) it holds an estimated 18,900 cubic kilometres (4,500 cu mi). It has an average surface temperature of 25 °C and a pH averaging 8.4.

The enormous depth and tropical location of the lake can prevent 'turnover' of water masses, which means that much of the lower depths of the lake is so-called 'fossil water' and is anoxic (lacking oxygen). The catchment area of the lake covers 231,000 km², with two main rivers, flowing into the lake, numerous smaller rivers and streams (due to the steep mountains that keep drainage areas small), and one major outflow, the Lukuga River, which empties into the Congo River drainage.

The major river that flows into this lake, beginning 10.6 ka, is the Ruzizi River, entering the north of the lake from Lake Kivu. The Malagarasi River, which is Tanzania's second largest river, enters the east side of Lake Tanganyika. The Malagarasi is older than Lake Tanganyika and before the lake was formed directly drained into the Congo river.

The lake's high altitude, great depth, slow rate of refill and mountainous location in a turbulently volcanic area that has undergone changing climate have all given the lake a complex history of changing flow patterns, with its outflow to the sea apparently historically unusual: it has been described as 'practically endorheic' for this reason. The lake's connection to the sea is dependent on a high water level allowing water to overflow out of the lake through the Lukunga into the Congo. Because of rapid evaporation from the tropical location of the lake, this in turn depends on a high inflow through the Ruzizi out of Lake Kivu to keep the lake high enough to overflow. This outflow is apparently not more than 12,000 years old, caused by lava flows blocking and diverting the Kivu basin's previous outflow into Lake Edward and then the Nile system, and causing Tanganyika's water level to rise up to its current overflow level: ancient shorelines indicate that at times Tanganyika may have been up to 300m lower than its present surface level, with no outlet to the sea. Even its current outlet is intermittent, and may not have been operating when first visited by Western explorers in 1858.

The lake may also have at times had different entries and exits: inward flows from a higher Lake Rukwa, access to Lake Malawi and an exit route to the Nile have all been proposed to have existed at some point in the lake's history.

Because of a lack of major rivers flowing into the lake to naturally replenish it, concern exists that any rise in temperature and evaporation through climate change could cause an extremely rapid collapse in the lake's level.