Lake Chad

Geology and physiography

The Chad basin is a downwarped section of the Precambrian African Shield. Most of the older crystalline rocks are covered by more recent deposits. The most significant physiographic influence on the basin was the ancient sea. At its maximum extent the sea was more than 600 feet (180 metres) deep, occupied an area of approximately 154,400 square miles (400,000 square km), and drained into the Atlantic Ocean through the Benue River system. It experienced four high stages between 41,000 and 2,300 years ago. The history of the sea is documented in the stratigraphic record, which includes thick layers of diatomaceous earth, lacustrine sands, terraced shorelines, and the remains of modern fish and mollusks in now-arid tracts of the basin. The floor of the basin dips to the northeast of the modern lake, reaching its lowest point in the Djourab Depression, some 300 miles (480 km) away. Lake Chad occasionally overflows into the generally intermittent El-Ghazal River leading into the depression, but it is usually confined by the dune fields of Kanem.

Climate

The climate of the Lake Chad region is strongly influenced by the seasonal migration and interaction of the dominant air masses of the region: a dry, subsiding continental air mass and a humid, unstable maritime air mass. The humid air mass moves northward during the summer, wedging beneath the drier air mass. Precipitation occurs when the depth of humid air is sufficiently great. The depth of the air mass varies daily as well as seasonally, accounting for variation in precipitation levels. At the end of the summer the dominance of the dry continental air mass is reasserted. Evaporation and transpirational losses from soil and plants increase, and then they decrease as the surface layers of the soil dry and plants lose their leaves. The dry season is also the period of the harmattan, a dust-laden wind that reduces visibility for days at a time. The increased insolation, reduced humidity, and desiccating winds contribute greatly to water loss in the lake.

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Precipitation levels are greatest from July to September. Annual precipitation averages 22 inches (560 mm) at the southern margin of Lake Chad and about 10 inches (250 mm) at the northern margin. Variability during the year is high and increases from south to north; variability from year to year is also high, and droughts are frequent. Temperatures during the wet season are moderate, with highs in the low 90s F (low 30s C). In October and November, during the transition to the dry season, daily highs rise above 90 °F (32 °C), and diurnal ranges are almost double those of the wet season. During December and January daytime highs are lower, with nighttime lows sometimes falling to the high 40s F (about 8 °C). April is usually the hottest month of the year, with temperatures occasionally reaching the low 110s F (low 40s C).

Hydrology

Lake Chad is a variable body of water. Its measured surface area typically fluctuates seasonally, peaking in late October or early November, then shrinking by more than half by late April or early May. The lake is dotted with numerous islands, which can coalesce into larger land areas during periods of extremely low surface levels. The volume of the lake reflects local precipitation and the discharge of its catchment area, balanced against losses through evaporation, transpiration, and seepage. The lake is fed chiefly by the Chari (Shari)-Logone river system, which accounts for about four-fifths of the inflow. Of the remaining inflow, most is contributed by the Ebeji (El-Béid) and Yedseram rivers. Losses to evaporation and the transpiration of aquatic plants amount to approximately 100 inches (2,500 mm) each year. It is probable that up to another 10 inches (250 mm) replenish groundwater supplies in the adjacent Manga and Kanem lowlands and pass as underflow through the El-Ghazal.

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Water and its Varying Forms

At times when the lake has a greater surface area, it can be divided into two pools partially separated by a low ridge extending roughly northeast-southwest across the centre of the lake; the ridge was formed during a drought at the beginning of the 20th century, and at times it has completely divided the basins. Typically, depths of 13 to 23 feet (4 to 7 metres) are common in the northwestern pool, and 33-foot (10-metre) depths can occur among the islands along the eastern margin of the pool. Because of sediment deposition by the Chari River, the southeastern pool is generally shallower—10 to 13 feet (3 to 4 metres) deep—but sometimes reaches depths of 36 feet (11 metres) along the archipelago. The gentle slope of the lakeshore allows persistent dry-season winds to locally affect water levels for short periods of time. The salt content of Lake Chad is unusually low for a tropical dryland lake with no outlet. As the waters of the lake evaporate during the dry season, the salt content increases, with the highest values recorded along the northeastern shoreline.

Travelers reported high water levels and overflow into the El-Ghazal during the 13th and 19th centuries. In 1870, for example, Lake Chad covered some 10,800 square miles (28,000 square km). At the turn of the 20th century the lake began to diminish in size, but by the 1920s it had recovered, and in 1956 it again overflowed into the El-Ghazal. During the 1970s and ’80s the amplitude of the lake’s annual variability was the highest recorded in the 20th century, with average levels falling below long-term norms; the surface area was reduced to less than 1,500 square miles (3,900 square km) for a time in the mid-1980s and again in the early 21st century. The corresponding variability in rainfall appears to have been related to the effects of environmental degradation.

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