What’s the dam truth?

By Gabrielle Jawer

While hydropower is affordable, dependable, and our largest renewable energy source, a recent report from the Virginia Scientist Community Interface (V-SCI) reinforces prior data that certain dams can be unusually high emitters. The report examines greenhouse gas emissions (GHGs) from two hydropower reservoirs in Alabama and found that both released more than a proposed clean energy threshold.

In what circumstances can this happen and how can we temper such emissions?

First, let’s examine how hydropower begets GHGs. Imagine a river. It’s autumn, and candy-colored leaves dance in the wind, floating down into the frothy water. In a free-flowing river, a leaf is a lunch for larvae and other organisms. These creatures shred the foliage into tiny particles which can be taken up by plants or eaten by bacteria. In this way, the leaf transforms into nutrients and carbon dioxide.

In a dammed river, a leaf can’t break down like that. Instead, it reaches still water and sinks. It drifts down into the dark cold waters where oxygen is scarce, and it settles in the mud. There, bacteria munch away and, because there’s no oxygen, the bacteria’s munching produces methane, a greenhouse gas with 25 times the warming effect of carbon dioxide.

‘But wait!’ you say. ‘Under that logic, ALL lakes must produce methane.’ Well, that’s right! Hydropower reservoirs aren’t the only ones that release methane. So do Lake Michigan, Lake Tahoe, and Walden Pond — all water bodies, to some extent. The quantity of methane produced depends on many things - temperature, water depth, and sunlight, to name a few. And not all of that methane reaches the atmosphere. Some stays in the mud, some diffuses into the water, and some bubbles up and into the air.

Here’s the rub: hydropower reservoirs convert rivers into lakes, producing lots of methane where previously there was little. Normally, methane bubbles up from the bottom or slowly diffuses at the surface, but with hydropower, it can also discharge while water tumbles through hydroelectric turbines. However, the processes of methane production and release are nuanced and highly variable, so it can be tricky to quantify methane discharge from a lake or reservoir.

V-SCI’s study evaluated the GHG emissions of Weiss and Harris Reservoirs using an open-source tool called G-res. G-res is an industry-standard modeling tool developed by the International Hydropower Association. The webtool performs complex calculations based on the GHG measurements of 223 worldwide reservoirs, but it functions like an online spreadsheet. Enter data about the reservoir and out comes an estimate of its annual emissions. “We weren’t sitting out there on the lake taking measurements, but it’s still an accurate and useful model,” confirms Stephanie Sickler, one of the graduate students behind V-SCI’s report.

Aided by G-res, V-SCI concluded Weiss Reservoir’s emissions were comparable to the 2020 average of emission intensity from Alabama coal and gas plants. Though less intense, GHG emissions from Harris Reservoir were 1.5 times greater than the proposed Clean Energy threshold.

How accurate are these results? Something called a sensitivity analysis illuminates error in the G-res model. Researchers adjust key input data and see how model outputs shift. For example, what happens if the reservoir is 10% deeper than we think? For most parameters, results were robust. But for the most sensitive parameters (reservoir size and global horizontal irradiance, i.e. how much sunlight the reservoir receives), results changed by up to 50%.

Even if our models are imperfect, they yield meaningful insights. Sickler adds: “Estimates [for Weiss and Harris] are so much higher than the threshold for the clean energy requirement, that even if they’re wildly off, they’re still way high.”

Fortunately, a four-year study is underway to better understand hydropower emissions. This massive survey by the United States Environmental Protection Agency is examining 108 reservoirs from 2020 through 2024, and the data may help inform policy and build better models. 

Where does this leave us? Should we scrap hydropower altogether? 

Sam Bickley is the lead author of V-SCI’s report. “You don’t have to vilify hydroelectricity because you can manage them to be cleaner,” he affirms. For instance, you can draw from the oxygen-rich top waters instead of the methane-heavy bottom waters, so methane doesn’t get a chance to escape. You can coordinate dam locations, so reservoirs generate less methane to begin with. You can even use existing dams instead of building new ones. 

V-SCI, too, continues to augment hydropower research. G-res analyses are underway for three different reservoirs on the Black Warrior River in Alabama, including Lewis Smith Lake (the deepest reservoir in the state) and Bankhead Reservoir (a ‘run-of-river’ dam with a higher water flow rate and smaller impoundment).

Researchers must study thousands more reservoirs to shed light on the complex processes behind hydropower’s unseen emissions. If you live in an area powered by hydropower, ask your utility if they’ve used G-res to evaluate local dams. You can learn more about V-SCI’s work at their website.

Although hydropower isn’t always 100% clean, it’s an invaluable energy source and one vital to our arsenal of renewables. We need not denounce hydropower because - with sufficient science - we can reform reservoir management and temper emissions.   

*Determined using 80% allocation of emissions for hydropower generation