A fresh look at the environmental impacts of dams on an ecologically diverse and partially protected river in China found that small dams can pose a greater threat to ecosystems and natural landscapes than large dams.

Although large dams are generally considered more harmful than their smaller counterparts, 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 hydropower sector.

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 as part of her PhD research while 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."

She and Desiree D. Tullos, also a water resources engineer at Oregon State and Kibler's PhD advisor, report their findings in a paper accepted for publication in Water Resources Research, a journal of the American Geophysical Union.

Kibler now works as a researcher at the International Centre for Water Hazard and Risk Management in Tsukuba, Japan, and as an Associate Professor at Japan's National Graduate Institute for Policy Studies in Tokyo.

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 modeled 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 Tullos 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 kilometers of river bed dewatered, Kibler explained.

From its headwaters in the Tibetan Plateau, the Nu River flows through China, Myanmar (Burma) and Thailand. "While the number of small hydropower dams in operation or planned for tributaries to the Nu River is unreported," the authors note in this study, "our field surveys indicate that nearly one hundred small dams currently exist within Nujiang Prefecture alone."

Thirteen large hydropower dams are proposed for the main stream of the Nu River in Tibet and Yunnan Province in China. "No large dams have been built, but there have been reports that site preparations have begun at some proposed dam sites," Kibler said.

Environmental, social, and economic factors make the Nu River basin extremely sensitive to hydropower installations. In addition to supporting several protected species, the region is home to a large proportion of ethnic minorities and valuable natural resources, the authors report in the study.

Parts of the Nu River are also designated as a World Heritage site and the Nature Conservancy and Conservation International have delineated stretches of this river and its tributaries as biodiversity hotspots. But proposed hydropower projects are threatening these statuses, according to Kibler.

While large hydropower projects are managed by the central government, and both large and small hydropower projects undergo environmental impact assessments, decisions about small hydropower projects are made at a provincial or other regional level and receive far less oversight, Kibler and Tullos state in their paper.

Small dams in China "often lack sufficient enforcement of environmental regulations" because they are "left to the jurisdiction of the province," said Guy Ziv, lead scientist for the Natural Capital Project, an organization which develops tools to assess and quantify natural resources, and a researcher for the Woods Institute for the Environment at Stanford University. This study, he added, is "an important contribution to the field of natural resource management."

The lack of regulation paired with a dearth of communication between small dam projects in China allows for the impacts to multiply and accumulate through several dam sites, the study authors write.

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

Policies supporting growth in the small hydropower sector are often crafted at the national or international level, Kibler noted. For example, many of the small dams investigated in the new study were supported by the Kyoto Protocol, a 1997 agreement to reduce greenhouse gas emissions.

"The lack of comprehensive analysis regarding cumulative impact of small hydropower," Kibler said, "is a significant research gap with important policy implications."

Cumulative biophysical impact of small and large hydropower development, Nu River, China Authors: Kelly M. Kibler: Water Resources Engineering, Oregon State University, Corvallis, Oregon, United States.