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  • Matt Collins, left, a hydrologist with the National Oceanographic and Atmospheric Administration’s restoration center, and Adam Pearson, a Boston University graduate student, use photographs Wednesday to compare the flow of a section of the Souhegan River now and last year before the Merrimack Village Dam was removed in Merrimack.
  • A rope swing marks the spot of a former swimming hole on the Souhegan River in Merrimack.
Thursday, May 7, 2009

River sees vast changes after dam is removed

By KAREN LOVETT

Staff Writer

MERRIMACK - A stone’s throw downstream from the Chamberlain Bridge, a ragged, white rope dangles from a tree.

At one time, you could grab the rope, swing out and splash into a deep pool of water in the Souhegan River. If you tried that now, you’d swing out and plunk onto a swath of sand - far more crash-landing than cannonball.

That spot and a few others on the Souhegan River have taken on a whole new look since last summer’s removal of the Merrimack Village Dam. And it’s no wonder, considering the dam dictated the waterway’s contours for some 275 years.

Take it away, and you’ll get big physical and biological changes in the river, which have psyched up folks who will spend the next few years monitoring everything from sand grains to fish species.

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This video shows 10 months of time-lapse photography showing the removal of the Merrimack Village Dam, taken by the National Oceanographic and Atmospheric Administration (NOAA) Restoration Center. The pictures run from July 16, 2008 through April 15, 2009. The so-called “Dam Cam” will continue to take pictures through July.

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<i>Courtesy video by NOAAM</i>

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“We love this site,” said Matt Collins, a hydrologist with the National Oceanographic and Atmospheric Administration’s restoration center in Gloucester, Mass. “You’re watching the stream and channel evolution happen right before you in rapid time. Normally, we witness in decades what we’re now witnessing in a compressed period of time.”

Collins is overseeing a range of monitoring activities being done by different groups on just over a half mile of the river. The target area spans from upstream, just below Merrill’s Marauders Bridge, to downstream, below the Chamberlain Bridge.

Collins said the level of post-dam removal monitoring being done is unique.

Before this project, scientists largely presumed dam removals were beneficial to the surrounding ecology, but they didn’t have a lot of data to back that up, Collins said.

“The public has asked, ‘Well, what really happens?’ ” Collins said. “We’ve been fairly embarrassed in just saying, ‘It’s great.’ The public can be more sophisticated than that . . . This will give us the tools to answer the public in specific ways.”

A few years ago, NOAA developed a guide that outlines eight specific monitoring approaches, which include water quality tests, changes in vegetation, bottom-dwelling bugs and fish.

A particularly important approach deals with cross-sections, which are surveyed points that measure width and depth from bank to bank.

Adam Pearson, a Boston College graduate student who is studying the changes in the river, and others previously set up 12 of them along the monitoring area.

Each time they remeasure a cross section, they record changes in widths and depths. These show how the river has carved out sediment in some places and dropped it off in others, particularly after the removal and during the heavy rains late last summer and fall.

At least three parts of the monitoring stretch have morphed significantly since that time.

An area upstream, just short of the Merrill’s Marauders Bridge, used to be “like a sandy bathtub,” Collins said. With the dam removal and subsequent “pulses” of water in the fall, the area is now defined by rock.

Collins described it as a “cascading mountain stream environment.”

Just downstream from there, but still short of the former dam site, is an area where an island had split the river into three channels. It was once a static spot, Pearson said, where the wind would push paddlers more than the current.

“Now, it’s a different flow,” he added.

Heavy erosion along one bank felled a couple big white pine trees right into the water, which have helped create a new, meandering “curly-cue” pattern in the current - an exciting find for the monitoring team.

Just over the island, where a channel once flowed, there is nothing but beach. Pearson said there’s now a kind of “race” to see whether the spot will vegetate and stay firm, or be overcome with water.

And finally, the area downstream from the Chamberlain Bridge - where the rope swing is - was simply “swamped with sand,” Collins said. Twice since the very wet late summer and fall, the river has changed course in that spot.

Pearson plans to conduct further cross-section analysis in June and August, and Collins said monitoring efforts should continue for years to come.

Fisherman may also be interested in what kinds of catch may be inhabiting the waters. There’s no data on it yet because the state’s department of environmental services will be sampling for fish species this spring and summer, comparing what they find to data collected before the removal.