The Dam Truth: Essential Answer

January/February 2013

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The Dam Truth: Essential Answer

Photo: Bsterling/Flickr

Q: How do dams impact watersheds and native wildlife?

M. Stoecker, Portola Valley, CA

Most of us have played with rubber duckies in the bathtub (some of us more recently than others). If you dropped a rubber duck in a newly dammed river, what would he see on his journey downstream from the source? Let's join our rubber duck and experience the upstream and downstream consequences of this dam, both on watersheds and wildlife.

First, we arrive at the dam's reservoir. Picture, for example, the placid surface of Searsville Lake, the reservoir of Stanford's Searsville Dam. Now look beneath the water: There are masses of plant matter. The terrestrial habitat that once existed here has been inundated and the vegetation is beginning to decay, releasing large amounts of carbon to the atmosphere. One study found that large dams around the globe release approximately 115 million tons of methane, a powerful greenhouse gas, every year. How much is that? Global methane emissions from livestock are considered to be one of the biggest causes of climate change, outside of fossil fuels. But in year 2006, ruminant livestock emissions were 95 million tons per year—80 percent as much as global emissions from large dams.

As we examine the reservoir, we notice the water level dropping ever so slowly. A reservoir is usually wide and shallow, exposing more of the water's surface area to the air than a river does. That boosts evaporation. Lake Nasser, the massive reservoir of the Aswan Dam in Egypt, loses about 10 percent of its volume each year to evaporation. This evaporation contributes to higher salt content in the remaining water, which can be toxic to aquatic plants and animals. It is also toxic to farm plants, making the water less useful for irrigation. Dam reservoirs also disrupt the natural balance of rivers by warming the water, and jacking up nutrient levels and sucking out oxygen as plant material decays. Our rubber ducky probably noticed some fish swimming past him during the journey into the reservoir, but he’s not likely to spot many lunkers as the oxygen levels dip. Dams also disrupt migration routes for trout and salmon, among other fish, leading to population decline, and even extinction.

With a big whoosh, we pass through the dam, tumble through a hydroelectric generator and move downstream of the structure. When the dam was not present, erosion and sedimentation in the river were in balance—new silt and sand drifted down to replace the material carried away. Now, though, the dam traps silt and other materials carried by the water. Over the years, the downstream channel is gouged deeper and narrower because the riverbed sediment is not replenished. The water table will dip lower underground, drying out wells and decreasing the amount of irrigation water available to surrounding areas. Farther down at the mouth of the river, the lack of replenishing sediments will also lead to coastal erosion. Water will whoosh out to sea faster, instead of meandering along and depositing sediment in a wetland-rich delta as it once did.

The dam we visited might have been built to generate electricity, to create reservoirs for irrigation and drinking water, to mitigate the impact of floods, or simply to produce a lake for recreation. Read on to the Nitty-Gritty answer for some case studies, from Stanford's Searsville Dam to a hydropower project in Iceland. Dams and their impacts are complicated, and no two situations are identical. If you feel strongly about dams—whether you want more dams and more fossil-fuel free energy sources, fewer dams and less ecosystem disruption, or simply less disruptive dams—there are organizations you can join to make your voice heard. Because from plants to fish to rubber duckies, dams affect us all.

Tracy Mandel is a graduate student in civil and environmental engineering.

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