Dam Drawdown an Overlooked “Global Warming Culprit”

 
While scientists have known for decades that tropical reservoirs are a significant source of greenhouse gas emissions, new research from Washington State University-Vancouver has revealed that temperate reservoirs can produce a significant surge in emissions during certain times in their operation. These periods are known as drawdown periods, when the water level in a reservoir drops rapidly, thereby exposing a “drawdown zone” of decayed plants that can be a continuous source of methane.

Marshes in the drawdown area of the Three Gorges Reservoir could be a significant source of methane. Photo via Wikimedia Commons

Bridget Deemer, the researcher leading the study, measured dissolved gases in the water column of Lacamas Lake in Clark County and found methane emissions jumped 20-fold when the water level was drawn down. A fellow WSU-Vancouver student, Maria Glavin, sampled bubbles rising from the lake mud and measured a 36-fold increase in methane during a drawdown. Deemer and Glavin will present their findings at a poster session at the national meeting of the Ecological Society of America in Portland this week.
 

 
While emissions from drawdown regions have long been recognized by researchers and international research bodies like UNESCO (see UNESCO and International Hydropower Association’s Greenhouse Gas Measurement Guidelines), according to John Harrison, Deemer and Glavin’s advisor and an assistant professor of Earth and Environmental Sciences, this is the first study to actually demonstrate and quantify the relationship between water-level drawdowns and greenhouse gas releases.

Fallen tree at Lacamas Lake, Washington
Photo: Lonie Lougheed

This is also one of the few studies we’ve seen to actually examine drawdown zones in temperate reservoirs. Drawdown emissions have been studied and modeled in the tropical context (see Fearnside, 2009 and 2005) and to a limited extent at the Three Gorges Dam (see Chen, H. et al., 2009). In the case of Three Gorges, for instance, one-third of the reservoir is a drawdown region and given its massive size (its surface area is the size of Hong Kong) — that is no insignificant source of methane. While dam reservoirs cover a small portion of the earth’s surface, as Harrison notes, they harbor biological activity that can produce large amounts of greenhouse gases. When you think of the number of large dams in the world – more than 54,000 that are over 15 meters – and the countless others that are being proposed or are under construction, continuing to overlook reservoirs as a carbon source and treating dams as a “carbon neutral” energy source is no longer a viable option.

To fix this situation, an important first step is for governments and dam builders to recognize that dams have a carbon footprint (and a potentially significant one depending on where it’s located, its age, depth, and organic inputs). Next, countries must report their reservoir emissions in their national greenhouse gas inventories. However, since the requirement is that they follow IPCC guidelines, the IPCC must first adopt strong guidelines like those developed by UNESCO/IHA. Only then can we get a true accounting of a country’s overall carbon emissions.

On the project scale, this research could have important implications for how dam operators manage drawdowns, as emissions may be higher in summer months when warmer temperatures and low oxygen conditions in bottom waters stimulate the microbial activity that produces greenhouse gases. Managers can also consider the optimal time to take out a dam, according to Deemer. While a dam removal may lead to some greenhouse gas emissions initially, it will be a one-time occurrence, whereas emissions can recur with regular drawdowns. With this in mind, Deemer plans to look at three other reservoirs in Oregon and northern California’s Klamath basin, where a major dam decommissioning effort is underway.