PNNL

As a result of microbial processes in waters and soils, all freshwater lakes and reservoirs emit gases such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), which can influence global climate processes. Considerable scientific debate continues about the extent to which these gases are emitted by reservoirs—especially across different regions, reservoir typologies, and highly variable temporal and spatial conditions. With tens of thousands of reservoirs across the United States providing flood control, navigation, irrigation, recreation, and hydropower generation, determining the scale of reservoir emissions is an important scientific endeavor.

The Past Bubbles Up

Scientists are discovering that large bodies of water, including rivers, streams, and reservoirs, emit greenhouse gasses into the atmosphere. One of these greenhouse gases is methane. Methane gas is created when carbon is introduced from fresh water sources upstream, and is then transformed into methane by microbes as part of the natural decay process of plant matter. Methane gas “bubbles up” and is released from the water’s surface. However, the type and amount of emissions released by large bodies of water varies widely depending on location, climate, and other factors.

A Closer Look at Two Columbia Basin Reservoirs

To better understand reservoir emissions, researchers from PNNL conducted a literature review and found numerous studies demonstrating that reservoirs around the world produce varying amounts of methane gas. Researchers then went a step further and studied the methane emissions of two U.S. hydropower reservoirs: Lower Monumental and Priest Rapids Reservoirs.

The team gathered samples around each of the reservoirs and nearby sites during the month of September in 2012. They found that 97 percent of daily methane emissions originated from just 22 percent of the surface area in Lower Monumental reservoir, and only 6 percent the surface area in Priest Rapids reservoir. The results showed that emissions from these reservoirs were within the range of other temperate hydropower reservoirs studied throughout the world.

Because there have been relatively few studies and results vary widely, there is a need for more research to accurately assess emissions from reservoirs. This type of information will help environmental managers and policy makers make informed decisions. For more information about the study read the team’s paper “Methane ebullition in temperate hydropower reservoirs and implications for U.S. policy on greenhouse gas emissions” in Environmental Management.