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Appendix C
Comparison of Different Power-Generation Methods 259

typically much warmer than the pre-dam water, which can change
aquatic faunal populations, including endangered species, and prevent
natural freezing processes from occurring. Some hydroelectric projects
also use canals to divert a river at a shallower gradient to increase the
head of the scheme. In some cases, the entire river can be diverted, leav-
ing a dry riverbed. Examples include the Tekapo and Pukaki Rivers.
A further concern is the impact of major schemes on birds. Since
damming and redirecting the waters of the Platte River in Nebraska for
agricultural and energy use, many native and migratory birds, such
as the Piping Plover and Sandhill Crane, have become increasingly
endangered.

Greenhouse Gas Emission
The reservoirs of power plants in tropical regions might produce sub-
stantial amounts of methane and carbon dioxide. This is due to plant
material in flooded areas decaying in an anaerobic environment, and
forming methane, a very potent greenhouse gas. According to the World
Commission on Dams report, where the reservoir is large compared to
ptg
the generating capacity (less than 100 watts per square meter of surface
area) and no clearing of the forests in the area was undertaken prior to
impoundment of the reservoir, greenhouse gas emissions from the reser-
voir may be higher than those of a conventional oil-fired thermal gener-
ation plant. These emissions represent carbon already in the biosphere,
not fossil deposits that had been sequestered from the carbon cycle.
In boreal reservoirs of Canada and Northern Europe, however, green-
house gas emissions are typically only 2 percent to 8 percent of any kind
of conventional fossil-fuel thermal generation. A new class of underwater
logging operation that targets drowned forests can mitigate the effect of
forest decay.

Population Relocation
Another disadvantage of hydroelectric dams is the need to relocate the
people living where the reservoirs are planned. In many cases, no
amount of compensation can replace ancestral and cultural attachments
to places that have spiritual value to the displaced population.
Additionally, historically and culturally important sites can be flooded
and lost. Such problems have arisen at the Three Gorges Dam project in
China, the Clyde Dam in New Zealand, and the Ilısu Dam in
Southeastern Turkey.