Researchers have documented the role dams play in global warming and the surges of greenhouse gases as water levels go up and down.
Bridget Deemer, doctoral student at Washington State University (WSU)- Vancouver, Canada, 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, according to a university statement.
Methane is 25 times more effective than carbon dioxide at trapping heat in the atmosphere. And while dams and the water behind them cover only a small portion of the earth's surface, they harbour biological activity that can produce large amounts of greenhouse gases.
There are also some 80,000 dams in the US alone, according to its Army Corps of Engineers National Inventory of Dams. "Reservoirs have typically been looked at as a green energy source. But their role in greenhouse gas emissions has been overlooked," said Deemer.
Deemer and Glavin's findings will be on display this week in a poster session at the national meeting of the Ecological Society of America in Portland.
Their efforts are part of a larger attempt to appreciate the role of lakes, reservoirs and streams in releasing greenhouse gases.
A study published last year in the journal Science conservatively estimated that the ability of terrestrial ecosystems to act as carbon sinks, storing greenhouse gases, could be one-fourth less than estimated once emissions from reservoirs are considered.
The WSU-Vancouver work is the first to actually demonstrate and quantify the relationship between water-level draw downs and greenhouse gas releases, said John Harrison, Deemer and Glavin's advisor and WSU assistant professor of Earth and Environmental Sciences.