Dams -- what goes up must come down, and then what?

November 7, 2011 By Miles O' Brien and Jon Baime

Time can take its toll on a dam. As dams age, they are more costly to repair and the risk of a catastrophic dam break increases--putting property and lives at risk. But, removing them can mean big changes to the community, and the environment.

"A lot of communities now are trying to wrestle with the decision of whether or not to support removal. And part of that uncertainty is our lack of of what's going to happen when you take a dam out," says Dartmouth College geographer Frank Magilligan.

The video will load shortly.

With support from the National Science Foundation (NSF), Magilligan studies river systems to learn how dam removal might affect them. His "lab" has been the relatively small Homestead Dam along the Ashuelot River in Swanzey, N. H., 60 miles south of his campus office. The Homestead Dam was built more than 200 years ago along the Ashuelot. It's long outlived its purpose, once serving as a for a local mill that is long gone.

When Magilligan heard the dam was going to be taken down in July 2010, he rushed with a team of researchers to the dam site. "We were really fortunate because we were able to get in several months before the dam came out to get all the necessary pre-removal data," says Magilligan.

One set of data is LIDAR imagery of the Ashuelot. LIDAR uses a laser-based ranging tool mounted on the bottom of a small plane. It can peer beneath vegetation showing the Ashuelot's former riverbed from centuries ago.

"LIDAR is a very sophisticated . What [we're] able to do is pick up very detailed, centimeter-scale topographic ," says Magilligan. LIDAR can get those in just a few hours, giving a more accurate snapshot of the river's flow at any given point in time. It's the kind of work that might take months to complete by ground surveillance.

A year after the dam's removal, the team has now returned to the Ashuelot to get new LIDAR measurements to pinpoint where the river is currently flowing.

"We'll be able to document a topographic snapshot before the dam was removed and a topographic snapshot a year after the dam was removed," says Magilligan. LIDAR doesn't penetrate water, so grad student John Gartner resorts to a little help from a GPS device.

"As part of the GPS analysis we're able to get centimeter-scale topographical information," explains Magilligan, who also studies the riverbed sediment to track how the path of the river is changing. Magilligan finds there have been notable differences since the dam's removal. "What we have seen from some of our field analysis is that there's been a couple of feet of bank erosion in some places. In other places, we see a couple of feet of bank deposition as well," he notes.

For Magilligan, it's all about "shoring up" what we know about how rivers flow, in order to make smart choices when it is time for a dam to come down.

Explore further: Dam removal increases property values

Related Stories

Dam removal increases property values

April 17, 2008

Two new studies appearing in Contemporary Economic Policy explore the impact of dam removal on local property values and find that property values increase after dams are removed.

Three Gorges Dam shrinking Yangtze delta

May 21, 2007

Chinese scientists have determined how China's Three Gorges Dam -- the world's largest dam -- affects downstream sediment delivery in the Yangtze River.

Beaver dam partly contains Canada oil spill

May 5, 2011

An oil spill near the native village of Little Buffalo in Canada's Alberta province was partly contained by a beaver dam, a provincial environment official said Wednesday.

Building a better dam map

June 2, 2011

Humans have been building reservoirs and dams for thousands of years. Over the past few decades, their construction has spiked as our need to harness water – critical in flood control, irrigation, recreation, navigation ...

China's great dam in midst of eco-debate

November 19, 2007

The year-old Three Gorges Dam along the Yangtze River has spawned environmental problems such as water pollution and landslides, Chinese officials admitted.

Model analysis helps protect river's ecosystem

March 6, 2008

The Grand Canyon will be experiencing a spring of yesteryear, as water flow rates from the Glen Canyon Dam will be significantly increased, then throttled back in a high-flow experiment that runs March 4 through 9. The result ...

Recommended for you

Scientists determine source of world's largest mud eruption

October 17, 2017

On May 29, 2006, mud started erupting from several sites on the Indonesian island of Java. Boiling mud, water, rocks and gas poured from newly-created vents in the ground, burying entire towns and compelling many Indonesians ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.