Environmental Science, 10th Edition (Cunningham)

Chapter 16: Air Pollution

GE Exercise: Lake Chad

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Overview: Lake Chad, p. 353

This broad, shallow lake was once one of the largest in Africa. In the 1960s, it was about the size of North America's Lake Erie. Persistent drought, however, coupled with withdrawals of irrigation water for agriculture reduced the lake surface area by 95 percent. Villages that were once on the lakeshore are now far inland and cut off from the lake by many km of desert. This is a tragedy for many native people who once depended on the lake for water and much of their food. Note the sand dunes piled up on the former shore by desert winds.

The phenomenon of drying of Lake Chad isn't new, however. During the last ice age 80,000 years ago Lake Chad covered 400,000 km2 (154,000 mi2), or roughly the present size of the Caspian Sea. At that time, the Sahara was a verdant savanna, with crocodiles, hippopotamuses, elephants, and gazelles. As the glaciers melted and the climate of northern Africa became warmer and drier, the lake shrank to about 25,000 km2. The lake's shallow waters (current average depth of 4 m) have long been sensitive to drought.

The former bed of the Ice Age Lake Chad is now a broad valley called the Bodélé Depression. Strong easterly winds whip up the silt sediments left behind by receding waters. During a dry winter, the depression produces 700,000 tons of wind-borne dust every day. This dust can be carried all the way across the Atlantic Ocean. Atmospheric scientists calculate that every year 40 million tons of dust from the Bodélé fall on the Amazonian forest in South America. This is more than half of the annual transport of dust to the Amazon and is thought to be the main source of minerals that sustain the forest.

1
What countries border what remains of Lake Chad today?
A)Sudan and the Central African Republic
B)Nigeria and Niger
C)Chad and Cameroon
D)Libya and the Congo
E)Mali and Rwanda
2
What additional countries once bordered Lake Chad? (The former lake is represented by the green area.)
A)Sudan and the Central African Republic
B)Nigeria and Niger
C)Chad and Cameroon
D)Libya and the Congo
E)Mali and Rwanda
3
What is the widest expanse of the lake today?
A)About 30 km
B)About 50 km
C)About 100 km
D)About 200 km
E)About 300 km
4
How long was the lake (NW to SE) when it filled all the green area seen today?
A)About 30 km
B)About 60 km
C)About 100 km
D)About 200 km
E)About 300 km
5
What are the darkest spaces between the dunes?
A)The dunes' shadows
B)Cloud shadows
C)Remnant ponds and small lakes
D)Scorched areas from slash and burn agriculture
E)Areas infested by swarms of black African bark beetles
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