Dams: What Goes Up Must Come Down, and Then What? – Science Nation

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.

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