Small dams harm environment more than bigger dams?

Although large dams are generally considered more harmful than the smaller ones, a Chinese research team's survey warns otherwise

Small dams can pose a greater threat to ecosystems and natural landscapes than large ones, a new study in China has warned.

Although large dams are generally considered more harmful than the smaller ones, the research team's surveys of habitat loss and damage at several dam sites on the Nu River and its tributaries in Yunnan Province revealed that, watt-for-watt, the environmental harm from small dams was often greater - sometimes by several orders of magnitude - than from large dams.

Because of undesirable social, environmental, and political implications, the construction of large dams often stirs controversy. Current policies in China and many other nations encourage the growth of the small hydro-power sector, researchers said.

But, "small dams have hidden detrimental effects, particularly when effects accumulate" through multiple dam sites, said Kelly Kibler, a water resources engineer who led this study at Oregon State University in Corvallis.

"That is one of the main outcomes of this paper, to demonstrate that the perceived absence of negative effects from small hydropower is not always correct," said Kibler.

To compare the impacts of small and large dams, Kibler investigated 31 small dams built on tributaries of China's Nu River and four large dams proposed for the main stem of the Nu River.

She assessed the environmental effects of these dams in 14 categories, including the area and quality of habitat lost, the length of river channel affected, the amount of conservation land impacted, and the landslide risk.

Because information regarding large dams is restricted under the Chinese State Secrets Act, Kibler modelled the potential effects of the four large dams using publically-available information from hydropower companies, development agencies, and academic literature.

After evaluating data from the field, hydrological models, and Environmental Impact Assessment reports about the small dams, Kibler and Desiree D Tullos, also a water resources engineer at Oregon State, concluded that impacts of the small dams exceeded those of large dams on nine of the 14 characteristics they studied.

One particularly detrimental impact of the small dams observed in this study is that they often divert the flow of the river to hydropower stations, leaving several kilometres of river bed dewatered, Kibler said.

In order to mitigate the detrimental effects of small dams, there is a "need for comprehensive planning of low-impact energy development," researchers said.