Small Dams Can Add Up To Big Problems

Which do you think would be worse for the environment: big dams or small dams? OK, I wouldn’t be asking if there wasn’t an unexpected twist to the answer, and the unexpected twist is that new research suggests small dams have hidden effects that could make them a worse choice.

Two Oregon-based researchers are drawing this conclusion – which could have wider implications – after studying dam sites on the Nu River and its tributaries in the southern China province of Yunnan.

small hydro

A tributary of the Nu River is dry below a dam, its waters diverted by a hydropower project. (image via NSF/Kelly Kibler)

“Small dams have hidden detrimental effects, particularly when effects accumulate” through multiple dam sites, said Kelly Kibler, a water resources engineer who led the study while at Oregon State University. “That’s one of the main outcomes, to demonstrate that the perceived absence of negative effects from small hydropower is not always correct.”

The Nu River has been in the news recently. The New York Times reported last month that “the Chinese government stunned environmentalists this year by reviving plans to build a series of hydropower dams on the upper reaches of the Nu, the heart of a Unesco World Heritage site … that ranks among the world’s most ecologically diverse and fragile places.”

Environmentalist have been fighting hard to protect the Nu’s status as one of just two major Chinese rivers without dams. But the study by the Oregon team indicates that the lower-profile damming of the Nu’s tributaries – largely undertaken at a regional level and with little oversight – could be more damaging.

To compare the impacts, the team looked at 31 small dams already in place on Nu tributaries. They then used all the information they could get their hands on –  from hydropower companies, development agencies and the academic literature – to model the impact of four large dams on the Nu. On nine out of 14 counts, the effects of the small dams were greater. In particular, the researchers noted that for small dams, developers “often divert the flow of the river to hydropower stations, leaving several kilometers of river bed dewatered.”

The point of the research isn’t that large dams would be OK; instead, it’s that small dams, often seen as relatively benign, can’t be ignored.

“The lack of analyses of the cumulative effects of small hydropower,” Kibler said, “is a significant research gap with important policy implications.”

The study discussed has been accepted for publication in Water Resources Research, a journal of the American Geophysical Union (AGU). This story is based on information provided by the National Science Foundation.

Pete Danko is a writer and editor based in Portland, Oregon. His work has appeared in Breaking Energy, National Geographic's Energy Blog, The New York Times, San Francisco Chronicle and elsewhere.