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MILES O'BRIEN: Time can take a toll on a dam. As they age, they are costly to repair,
and the risk of a catastrophic dam break increases. But removing them can mean big
changes to the community and the environment.
FRANK MAGILLIGAN: A lot of these communities now are trying to wrestle with the decision
of whether or not to support dam removal. And part of that uncertainty is very much
hindered by our lack of scientific understanding of what's going to happen when you take a dam out.
Zero point three four. Got it.
MILES O'BRIEN: With support from the National Science Foundation, Dartmouth geographer
Frank Magilligan, studies river systems to learn how dam removal might affect them.
His lab has been the relatively small Homestead Dam, built more than 200 years ago along the
Ashuelot River in New Hampshire. This is what it looked like a year ago before the dam was removed.
FRANK MAGILLIGAN: 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.
MILES O'BRIEN: Data like this image of the Ashuelot, captured using a laser-based
ranging tool called LIDAR, which can peer beneath vegetation, showing the Ashuelot's
former river bed from centuries ago.
FRANK MAGILLIGAN: What they were able to do was pick up very detailed
centimeter-scale, topographic elevation.
MILES O'BRIEN: Now Magilligan is taking a set of post-dam LIDAR images to pinpoint
where the river is currently flowing.
FRANK MAGILLIGAN: We'll be able to document a topographic snapshot before the dam was
removed, and a topographic snapshot a year after the dam has been removed.
MILES O'BRIEN: LIDAR doesn't penetrate water. So grad student John Gartner resorts
to a little help from a GPS device.
FRANK MAGILLIGAN: As part of that GPS analysis we're able to get
centimeter-scaled topographic information.
MILES O'BRIEN: Magilligan also studies riverbed sediment to track how the path of the river
is changing. He's already detected notable differences since the dam's removal.
FRANK MAGILLIGAN: What we've also 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 up to a
couple of feet of bank deposition as well.
MILES O'BRIEN: 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.
For Science Nation, I'm Miles O'Brien.