Early on a warm September morning, I launched my canoe into the calmness of Weiss Lake in northeast Alabama to spend a day fishing and soaking up late summer lake life. As I paddled along the splayed fingers of the lake, I watched for the structure of submerged logs and stumps where fish congregate. Casting my line to a downed maple sagging in the water, I noticed a trail of bubbles emerge from the depths and silently burst on reaching the surface. The controlled exhalation of some enormous amphibian? Throughout the day, similar packages of bubbles rose like balloons to the surface and released their gaseous contents to the atmosphere. These bubbles are now hard for me to ignore because I’ve learned they are exhalations of the lake itself, releases of methane that are contributing to the overheating of our planet.
Most Alabamians know that Weiss “Lake” is actually a man-made reservoir formed by the dam of the same name built by Alabama Power in the late 1950s. Throughout the twentieth century, hydropower projects dramatically remade the map of Alabama, inundating farmlands and valleys, displacing communities, and transforming once free-flowing rivers into deep, still reservoirs. While the dams brought electrification and economic prosperity to much of the state, these human-wrought ecosystem upheavals drove scores of Alabama’s aquatic species to extinction, and we are now increasingly understanding how these artificial water bodies have become significant sources of greenhouse gasses.
By damming rivers and flooding lands to form reservoirs, we have submerged massive amounts of carbon (stumps, soils, entire landscapes). Unlike naturally flowing healthy rivers, reservoirs often experience low- or no-oxygen conditions, and organic matter that decomposes in the absence of oxygen produces methane, a particularly problematic heat-trapping gas. Methane can then escape from reservoirs and enter the atmosphere by bubbling out of sediments, degassing as water is passed through a dam’s turbines, or dissolving into water and then diffusing at the water’s edge.
Hydro dams and reservoirs have long been considered a source of clean, renewable energy that doesn’t contribute to climate change. Most of us, including policymakers and regulators, bought into the “hydro is clean” narrative hook, line, and sinker. But over the last twenty years, a substantial body of scientific research points in the opposite direction; that not only can dams and reservoirs be a source of carbon emissions, some individual reservoirs actually emit methane and carbon dioxide on par with coal and natural gas.
With the help of volunteer science PhD students, Alabama Rivers Alliance analyzed the total emissions footprints of two Alabama Power reservoirs—Weiss on the Coosa River and Harris on the Tallapoosa River (also known as Lake Wedowee)—using the G-res Tool, a modeling tool for researchers and hydropower companies to estimate and report net emissions from reservoirs. We’ve made all of the reports and data available for public scrutiny. The web-based G-res model is free for personal use, and anyone can input data about a particular reservoir and see its emissions profile.
The emissions of these reservoirs are in line with the scientific literature but stunning nonetheless; our analysis shows both hydro reservoirs exceed the proposed clean energy threshold that would qualify a particular energy generation source as “clean” based on the amount of emissions produced per unit of energy. Harris reservoir emits at roughly 1.5x the clean energy threshold, and Weiss emits at nearly 5x the clean threshold, putting Weiss’ emissions profile on par with gas and coal. (Note the G-res validations cover the total reservoir carbon footprints and not the services allocations, which we computed using G-res’ methodology.) What is abundantly clear from the results is that before the dams, these landscapes and rivers stored carbon, and after impoundment, the hydropower facilities have become sources of carbon emissions.
Alabama Power’s corporate parent, Southern Company, plans to reach net zeroemissions by 2050 but is currently counting its hydroelectric facilities as “zero-carbon resources.” As the broader science and our specific analysis show, that’s just not true. Mischaracterizing its hydropower as zero-carbon will have significant consequences for the decisions that Southern Company, its regulators, and stakeholders are making now about what energy assets get built, retrofitted, or retired. Pretending hydropower is emissions-free skews the net-zero math.
We are calling on Southern Company to assess the emissions of its hydropower fleet and to publicly report that data. If the utility wants to label its hydropower as zero-carbon, it must prove it by (1) investing in technologies to monitor reservoir emissions and (2) publicly reporting the data. Better data leads to better decision-making, and once the extent of the problem is known, then Southern Company must act to reduce the emissions from its reservoirs. Mitigation opportunities exist: increasing oxygen levels, adding dam intakes in the upper well-oxygenated layer of reservoirs, changing operating patterns, reducing nutrient pollution into reservoirs, and capturing methane for use as energy are all possible solutions to reduce emissions.
I’m back on a bubbling Alabama reservoir for another day of fishing as the utility begins its fall drawdown, lowering the lake to its winter pool level. These annual water level drawdowns can cause even more methane to escape from reservoirs, as pressure fluctuations spur additional bubbling. If learning that what we have been told is emissions-free energy actually creates significant amounts of methane frustrates you, please take a moment to ask Southern Company to assess the emissions of its hydropower reservoirs, to share the data, and to tell us all the dam truth.