23 August 2012


There are more than 80,000 dams across the United States.  They are constructed for a variety of reasons, including the generation of hydroelectric power, stabilizing the water flow of a river, and assuring a reliable water supply for agricultural irrigation or for urban consumers.

Dams create serious problems, however.  The presence of such a substantial barrier interferes with the migration and spawning of native fish (notably salmon).  Similarly, commercial and recreational boat traffic is blocked.  The lakes and reservoirs which form behind dams drown both human communities and entire riparian ecosystems beneath hundreds of feet of water.  Further, by impounding a river's flow in the extensive reservoir behind a dam, then releasing the trapped water at a regulated rate, we create unintended consequences for downstream habitat and wildlife.

The most notorious example is Glen Canyon Dam in northern Arizona.  The dam spans the Colorado River gorge upstream from the Grand Canyon.  Historically, spring floods from the Colorado River watershed swept away rocky debris which had washed down from side canyons, and during the rest of the year the river deposited sediment which formed sandbars and islands.  This cycle created a unique ecosystem, with mammals, birds, fish and reptiles living in a community unlike any other on Earth.  The dam changed all that.  Water releases do not approach the flood conditions of spring waters, thus are unable to carry away the rockslides common within the Canyon.  Releases tend to be timed to the hydroelectricity needs of cities as far away as Phoenix and Las Vegas, without regard for the riparian ecosystem of the Canyon.  Further, sediment is trapped behind the dam (which is slowly but surely filling the bottom of Lake Powell), robbing the Canyon of that alluvial resource.  Lastly, water is released from the colder depths of the lake, further altering conditions downstream for native fish and wildlife.

In recent years a movement has arisen which seeks to remove many of the dams which cause more problems than they solve.  In addition to the problems described above, dams present an often-ignored but very real threat ~ the possibility of rupture or failure due to faulty design, poor construction, aging, extreme inflow from spring snow melt, or an earthquake.  (E.g., in June 1964, not one but three dams on the east slope of the Rocky Mountains in Montana breached, sending flood waters hundreds of miles downstream.  The scale of the disaster was staggering ~ thirty thousand square miles (roughly 20 percent of the state) were affected.  Highways, railroads, ranches, livestock and wildlife were swept away.  Thirty people lost their lives.)

A few days ago I came across an article with an unexpected title ~ The Effect of Dams on Global Warming.  Here is the gist ~  "There is a lot of biological activity going on behind the dam.  This is where the natural flow of sediment and living things stop and therefore accumulate.  Through the decomposition of such creatures and organic material comes methane gas that builds up in the lake bed.

"As levels of water go down, the lake bed heats up because more sunlight is hitting it [or more accurately, more sunlight is penetrating to what was formerly deeper water].  The rising temperatures cause the methane to bubble up and out into the atmosphere.  This is particularly true in summer because low oxygen conditions at the depth of the reservoir create an ideal condition for microbial activity that creates methane .... In a study of the water column at such a reservoir, marine scientists found an astonishing 20-fold increase in methane emissions as water levels were drawn down.  Bubbles coming out of the mud and sediment at the bottom were chock full of this potent greenhouse gas."

Dams as contributors to global warming.  Who knew?  It is a vicious cycle ~ more methane leads to more warming, which lowers reservoir levels, which leads to the release of more methane.  One more instance of our excesses coming back to haunt us.

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