From International Rivers

Typically, at moderate sizes, power generated by dams and reservoirs is considered “green.” However, a new study from Washington State University has found that during times of drawdown — a period in which the water level behind a dam is rapidly lowered — temperate reservoirs can produce up 20 times more methane than normal.

Methane is a greenhouse gas 25 times moreeffective than CO2 at trapping heat in the atmosphere over 100-year period, and is a hundred times more potent over 20 years. It is produced naturally in reservoirs thanks to biological activity.

During drawdowns, though, when layers of decaying plants, among other things, are exposed, the amount of methane in the water column skyrockets. According to the study:

“Bridget Deemer, a doctoral student at Washington State University-Vancouver, 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.”

Though researchers have long known that methane levels spike in reservoirs during drawdown, this study was the first to show the relationship and put a number on the actual methane emissions.

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