Page 422 - Environment: The Science Behind the Stories

in southern China (where water is plentiful) to northern We have erected thousands of dams
China’s Yellow River, which routinely dries up at its mouth
because the climate is drier and people withdraw much of A dam is any obstruction placed in a river or stream to block its
its water. Three sets of massive aqueducts, totaling 2500 flow. Dams create reservoirs, artificial lakes that store water for
km (1550 mi) in length, are being built to move trillions human use. We build dams to prevent floods, provide drinking
of gallons of water northward. China’s leaders hope the water, facilitate irrigation, and generate electricity (pp. 592–593).
diversions will solve water shortages for northern farms Worldwide, we have erected more than 45,000 large dams
and cities. Many scientists say the $62 billion project (greater than 15 m, or 49 ft, high) across rivers in over 140
won’t transfer enough to make a difference, yet would nations. We have built tens of thousands of smaller dams. Only a
cause extensive environmental impacts and displace hun- few major rivers in the world remain undammed and free-flow-
dreds of thousands of people. ing. These run through the tundra and taiga of Canada, Alaska,
In the past, large-scale diversion projects have enabled and Russia and in remote regions of Latin America and Africa.
politically strong yet water-poor regions to forcibly appropri- Dams produce a mix of benefits and costs, as illustrated
ate water from communities too weak to keep it for themselves. in Figure 15.16. As an example of this complex mix, we can
The city of Los Angeles grew by commandeering water from consider the world’s largest dam project. The Three Gorges
the rural Owens Valley 350 km (220 mi) away. In so doing, it Dam on China’s Yangtze River, 186 m (610 ft) high and 2.3
turned the environment of this region to desert, creating dust- km (1.4 mi) wide, was completed in 2008 (Figure 15.17a).
bowls and destroying its economy. Then in 1941, city leaders Its reservoir stretches for 616 km (385 mi; as long as Lake
decided to divert streams feeding into Mono Lake, over 565 Superior). This project provides flood control, enables boats
km (350 mi) away in northern California. As the lake level fell and barges to travel farther upstream, and generates enough
14 m (45 ft) over 40 years, salt concentrations doubled, and hydroelectric power to replace dozens of large coal or nuclear
aquatic communities suffered. plants.

40–80 million people New recreational
Reliable drinking displaced by dam projects opportunities on reservoir
water if watershed in past 50 years Sediment settles behind dams,
lands are protected filling reservoir and not nourishing
downstream floodplains

Small risk of
Reliable irrigation catastrophic failure
for farming

Habitat alteration
(upstream and
downstream)

Lost recreational
Carbon emissions opportunities
much lower than on river
power from fossil CHAPTER 15 • Fr E shwat E r s yst E m s and rE sour CE s
fuels

Fisheries declines from
thermal pollution and
blockage of migration
Generation of
renewable electricity

Disruption of flooding
Flood control by that builds topsoil
storing seasonal
surges
Figure 15.16 Damming rivers has diverse consequences for people and the environment. The
generation of clean and renewable electricity is one of several major benefits (green boxes) of hydroelectric
dams. Habitat alteration is one of several negative impacts (red boxes). 421

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