The Lower Snake River Reservoirs Generate Significant Amounts of Methane, a Potent Greenhouse Gas

The reservoirs formed by the lower Snake River dams produce significant amounts of methane (a potent greenhouse gas) — adding to the list of reasons why a hydropower is not “clean” energy. Since 2016, Dam Sense has been analyzing government (including Intergovernmental Panel on Climate Change, “IPCC”), and other scientific, data on the production of greenhouse gases by hydropower reservoirs.

Our 2016 paper “The Lower Snake River Reservoirs Generate a Significant Amount of Methane, a Potent Greenhouse Gas” reviewed scientific research on the topic including “Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis” (Biosciences, 2016) and “Evaluating Greenhouse Gas Emissions from Hydropower Complexes on Large Rivers in Eastern Washington” (Pacific Northwest National Laboratory, 2013) (measuring and comparing greenhouse gas emissions from Priest Rapids Dam Complex on the Columbia River reservoir and Lower Monumental Dam Complex on the Snake River against a free-flowing section of the Columbia River).

Based on these papers and data specific to the Lower Snake River (water temperature, surface area, velocity, trophic state, and surrounding land use), we initially estimated the yearly output of the Snake River Dams to be 50,744 metric tons of CO2e.

In 2020, based on a new report by the Pacific Northwest National Laboratory (PNNL) titled “Methane Ebullition in Temperate Hydropower Reservoirs and Implications for US Policy on Greenhouse Gas Emissions” (PNNL, 2017), which included research specific to Lower Monumental Reservoir, we published an updated Methane Paper concluding that “The total greenhouse gas emissions of the lower Snake River are the equivalent of 86,053 metric tons of CO2. For comparison, the PSE Encogen natural gas-fired plant in Whatcom County WA generates 165 MW according to PSEs website and emits 96,443 metric tons CO2, according to EPA’s GHG emissions sources website.”

Increasing water temperatures will only exacerbate this problem. As our 2020 paper explains: “Higher temperatures and fluctuating precipitation rates from global climate change are exacerbating wildfires, lowering water levels, decreasing land productivity, and accelerating the extinction of salmon and other keystone species, among other environmental concerns. Heat induced outcomes such as an increase in water temperatures above which is needed for algal growth, contribute to climate change in a positive feedback loop. As waters warm, alga have the potential to increase, increasing the release of methane from the water column where they decompose, contributing to a further increase in temperature. Between 1960 and 2015, water temperatures in the Snake River have increased by an estimated 1.4°F.”

Well aware of the climate crisis and these at-risk ecosystems, 55 scientists signed a letter to Pacific Northwest policymakers in October 2019 arguing that the only way to keep future temperatures down in the Snake is to breach their dams. Using a 2003 EPA climate model, the scientists explain that “the four lower Snake Dams could affect temperatures up to a potential maximum of 6.8° C/12.2° F.”

Over the next 50 years will hydropower reservoirs be contributing more CO2 to the atmosphere than remaining coal and natural gas plants? Could the hydro system become as unsustainable as fossil fuel-based forms of energy? If so, strategies to reduce GHG emissions will need to include dam removal and river restoration. An understanding of projected GHG emissions for underperforming dams, such as the four on the Snake, adds to the case for breaching. Certainly, the labels of clean and carbon free cannot accurately be used to distinguish this form of energy.