Sustainable Hydropower: Are Small Dams Really Better for the Environment?

Small hydropower dams (often defined as below 30 MW) are frequently presumed to be low impact.  However, this dam, Veazie, on the Penobscot River, had a capacity of 8.4 MW and was a major barrier to migration for the most important population of Atlantic salmon in the US. It was removed in 2013. Photo: ©Misty Edgecomb/TNC

Small hydropower dams (often defined as below 30 MW) are frequently presumed to be low impact. However, this dam, Veazie, on the Penobscot River, had a capacity of 8.4 MW and was a major barrier to migration for the most important population of Atlantic salmon in the US. It was removed in 2013. Photo: ©Misty Edgecomb/TNC

By Jeff Opperman, senior freshwater scientist

When it comes to dams, is small beautiful? Or are these dams tiny but terrible?

Economist E.F. Schumacher popularized the phrase “small is beautiful” to evoke appropriate technology that minimized social and environmental impacts.

When it comes to hydropower, Schumacher’s aphorism is often interpreted literally and taken as an article of faith.

It’s easy to see why: small hydropower generally evokes a quaint New England mill dam, in stark contrast to the behemoths—hydro dams such as Hoover, Kariba and Three Gorges—that change the face of the earth as viewed from space.

But does small hydropower warrant its reputation as being environmentally friendly? Beyond the elegant aphorism, what’s the evidence?

Answering that question is increasingly urgent. Globally, dozens of policies reflect an underlying assumption that small hydropower is “green.”

In the US, several states with Renewable Portfolio Standards disallow electricity from large hydropower dams but embrace power produced from small projects (defined inconsistently as a megawatt capacity less than 30, 50, or some other double-digit number), in part due to a perception that small equals low impact.

The Clean Development Mechanism, launched under the Kyoto Protocol and intended to reduce greenhouse gas emissions, promotes small hydropower and streamlines its review due to perceived lower environmental impacts.

Countries across the globe—from China and India to Brazil and Mexico—have passed policies promoting small hydropower and subject its development to far less planning and regulatory oversight compared to large projects.

In one respect the answer is obvious: a 10 gigawatt project is going to have different—and much larger—impacts than a 10 MW project.  It’s like comparing apples to…apple seeds.

But that is not a terribly helpful insight. To be more useful, comparisons require scaling impacts to the energy that the dams produce.

Small Dams: Tiny but Terrible?

A recent study (Kibler and Tullos 2013) looked at scaled hydropower impacts in the Nu River basin of southwestern China (the Nu becomes the Salween as it flows into Burma).  The researchers calculated impact per MW of capacity across 14 metrics between small and large hydropower projects (with small being below 50 MW, as defined in Chinese policy).

They found that small hydropower dams had greater impact per MW for 9 of the 14 metrics, including length of river channel affected and impact on habitat designated as conservation priorities.

These results were summarized in media reports as, “small dams can pose a greater threat to ecosystems and natural landscapes than large dams.”

Because their study site includes one of China’s last major rivers without a large dam across it, this isn’t necessarily the most appropriate, or helpful, headline from their study.

After all, a 9-5 score in “impact metrics” in the David(s) vs. Goliath battle of the hydros isn’t as straightforward as 9-5 in runs for baseball:  perhaps cutting a mainstem river in half, as a large dam can do, outweighs all other impact metrics, like catching the golden snitch in Quidditch (don’t laugh, I couldn’t think of another sport with a scoring event so disproportionate).

But, if nothing else, these results clearly indicate that small hydropower can’t get a free pass and be presumed to be environmentally benign.

By scaling impacts to power capacity, the results are relevant to a somewhat obvious problem with small hydropower projects: the cumulative impacts of their proliferation across a river basin or region.

But even more disconcerting than cumulative impacts are examples illustrating that even individual small hydropower dams can cause significant impacts.

In other words, just like two-year old children, small hydropower dams don’t require strength in numbers to be naughty; they can be tiny and terrible on their own.  And like an overtired toddler on a plane stuck on the tarmac, a small dam’s bad behavior usually results from being in the wrong place.

Location, Location, Location

You’ve no doubt heard a great deal about the breakthrough project on the Penobscot River that removed two hydropower dams and will decommission and bypass a third.

The numbers associated with this project are somewhat astounding, including the kilometers of river that will now be accessible to a dozen species of migratory fish and their projected population increases in response to that increase in habitat.

For example, fisheries biologists forecast that Atlantic shad will increase from a few thousand to over two million.

And what about the dams that were suppressing these millions of fish?

The size of these dams is just as astounding: 8.4, 7.7, and 1.9 MW.  That’s right, fish populations in the most important Atlantic salmon river in New England were dramatically depressed for a century by dams that are far below the cutoff often presumed to equal “low impact” (e.g., 30 MW).

A modeling exercise (Kuby et al. 2005) also illustrates the extreme naughtiness of tiny dams in the wrong place.

Assessing strategies to increase connected channel network for salmon in Oregon’s Willamette River basin, they compared removal options from among 150 dams in the basin.  They found that removing only 12 dams could reconnect 52% of the drainage basin with a loss of less than 2% of the basin’s hydropower and water-storage capacity.

This was a modeling exercise, so these 12 small dams remain, each having an outsized impact on river connectivity.

The take-home message? Whether small or large, it’s all about location in a river basin. 

Small hydropower dams can certainly be part of a sustainable hydropower system.  And so can large hydropower dams.

For both large and small, the key word is system: sustainable hydropower requires planning, siting, and operating dams within an overall system that seeks to balance a range of resources and values.

Without this system approach, even tiny dams can cause huge environmental impacts.  Clearly they can do so as a rambunctious crowd, but even sometimes as a solitary troublemaker.


Kibler, K. M. and Tullos, D. D. (2013). Cumulative biophysical impact of small and large hydropower development in Nu River, China. Water Resources Research, 49, 1-15.

Kuby, M. J., Fagan, W. F., ReVelle, C. S. and Graf, W. L. (2005). A multiobjective optimization model for dam removal: an example trading off salmon passage with hydropower and water storage in the Willamette basin. Advances in Water Resources, 28, 845-855.

Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.

Posted In: Freshwater

Jeff Opperman is The Nature Conservancy's senior advisor for sustainable hydropower. He works to promote ecologically sustainable water management in river basins with hydropower infrastructure. Through this work, Jeff has provided strategic and scientific assistance to environmental flow assessments for several rivers in the United States and for the Yangtze River and the Patuca River (Honduras).

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