September 3, 2009

Back to nature
What happens when you remove a dam? Now we know.
By Jeff Mucciarone jmucciarone@hippopress.com

In its simplest form, a dam is an impediment to any flow of water. It could be intended to create a pond, to control flow for energy or to offer flood protection. Take the impediment out, and the result, environmentalists say, can be a beautiful thing.

The Souhegan River is reaping the benefits of removing its biggest impediment, the Merrimack Village Dam, which had stood since the 1700s. The removal occurred about one year ago, and today the river is marked by fast and clear currents, massive boulders and plenty of fish — a setting that feels a lot more like the wilderness in the northern parts of the state, said Deborah Loiselle, who coordinates the dam removal and restoration program for the state Department of Environmental Services.

The $592,000 removal of the Merrimack Village Dam is the marquee accomplishment of the state’s removal program, which began in 2000. It opened a 14-mile stretch of river from Merrimack to Milford, potentially opening up historic fish runs to shad, salmon, and herring.

“We’re seeing a lot of exciting changes [on the Souhegan River],” Loiselle said.

The most significant change occurred upstream of the dam where a sandbar stretches to just below the Everett Turnpike bridge. That’s where the massive boulders and cobbles have become unearthed to dot the river: “It’s absolutely beautiful,” Loiselle said.

State biologists who sampled the same stretch of the Souhegan in August found hundreds of salmon. That means the salmon, which were stocked juveniles, now have the sandy substrate habitat they desire. Prior to dam removal there were no salmon, Loiselle said, and anglers are enjoying the new stretch for fishing.

Noah Snyder, a professor with Boston College who is leading a research team studying the changes in topography pre- and post-removal of the Merrimack dam, said the changes just in the last year have been remarkable. Since the removal, about half of the sediment build up has washed downstream and about one third washed away within the first couple weeks.

“This is an unusual experiment,” Snyder said. “It’s an opportunity to learn about river processes. We figured it would happen. Sand is easy to erode — it doesn’t stick to itself.”

Snyder said the river channel developed quickly where the dam once was and eroded the river bed right down to boulders and bedrock that were presumably exposed on the “pre-dam” river bed.

“It really looks like a good-sized river in southern New Hampshire,” Snyder said.

Things have gone pretty much as officials expected with the Merrimack removal, as well as with the Maxwell Pond Dam removal on Black Brook in Manchester. Unlike the Merrimack dam, the sediment behind the Maxwell Pond Dam was released gradually. That allowed amphibians and aquatic vegetation to stabilize as sediment and water levels dropped. That was especially important since it was done in winter, a time when many amphibians are burrowing.

The Maxwell Pond Dam was causing dissolved oxygen issues, issues so bad the waterway was placed on a government list of impaired streams. Almost instantaneously upon removal, the dissolved oxygen rose to much healthier levels, Loiselle said.

The Merrimack dam and the Maxwell Pond Dam represented the first obstructions on tributaries of the Merrimack River.

“That first obstacle, that’s pretty key,” said Steve Landry, Merrimack watershed supervisor with the state Department of Environmental Services.

The state’s role
The state isn’t looking to remove all 4,800 dams in the state. It’s offering removal as an option to dam owners who have seen dams — many of which no longer have any practical purpose — fall into disrepair. It can be a knee-jerk reaction to fix a dam, potentially an expensive reaction, whereas removing a dam is typically cheaper in the short and long terms, officials said. Not to mention that removal is a one-time cost.

Many dams today still have legitimate purposes, such as hydro power, water supply or recreation. It’s the ones not serving a purpose and falling apart — the ones that become financial and safety liabilities and are unnecessarily clogging a river’s flow — that officials have their eyes on.

Dam removal can be a lengthy and complex process. It took eight years, nine funding sources and a partnership of 16 entities to ultimately take down the Merrimack Village Dam, which was owned by Pennichuck Water. Removal is very much centered on educating the public. Particularly in Merrimack, where the dam was an icon, officials needed to tread lightly in explaining why removal made sense.

Officials in Milford are at the beginning stages of the possible removal of two dams, one owned by the town, the McLane Dam, the other owned by a miner from New Mexico, the Goldman Dam. The town is working to obtain funds for a feasibility study. Loiselle said the McLane Dam is not in immediate disrepair but could eventually prove costly in terms of maintenance. “We need to look at these dam removal projects individually,” Loiselle said. “Every project is different.”

The state is working on several other dam removal projects, including the Winnicut River Dam and the Homestead Woolen Mill Dam on the Ashuelot River.

Sediment pileup
Within hours of the breaching of the Merrimack dam, onlookers could witness the mass of sediment that had built up during the dam’s life slump and begin to wash downstream. The displaced sand did cause the river bed downstream to rise in the lower part of the river, closer to the Merrimack River, Snyder said.

“A lot piled up temporarily,” Snyder said. “The river’s been cutting back down into that. It’s presumably being sent out to the Merrimack River....”

One of the first things officials looked at prior to removal was sediment, to make sure it didn’t contain contaminants. Sand tends not to carry contaminants. Releasing sediment with harmful chemicals can be disastrous. Snyder pointed to a famous dam removal on the Hudson River in the 1970s, which released contaminants downstream causing major problems, problems the river still suffers from today. “That’s the cautionary tale,” Snyder said.

Snyder isn’t finding anything out of the ordinary downstream. Sand is dynamic, meaning it is constantly shifting and changing. New pools form and then fill in. The river channel continues to shift. “It didn’t fundamentally change the habitat downstream,” Snyder said.

Sediments tend to pile up at the mouth of the Souhegan River, where it enters the Merrimack River, a much larger waterway. When the Merrimack River is at a high flow, it actually backs up water into the mouth of the Souhegan River.

“The Merrimack is damming up the lower Souhegan, temporarily creating a lake,” Snyder said. “It’s a natural process and another reason the system is so dynamic.”

All streams naturally carry sediment, dropping it off at different points along the waterway. As Black Brook shifted sharply from a fast-moving stream to an area of slow-moving water, sediment dropped out, stockpiling behind the Maxwell Pond Dam, to the point where a 10-foot pond became three feet of stagnant water. The lack of movement, combined with constant sunlight and shallow depth, lead to an environment largely without oxygen, Landry said.

The state “de-watered” Maxwell Pond over two months, which allowed for the gradual release of sediment and let the water begin to eat through the layers of accumulated mud. “It’s moving constantly, eroding, digging down, widening, delivering more and more sand and gravel down through the system,” Landry said.

Landry said in a matter of a day or so, viewers could see boulders appear and ledge being exposed.

“It was being daylighted from … under a choking blanket of sediment,” Landry said. The stream will still continue to eat away at the sediment for the next couple years.

“The wet summer is really not a bad thing,” Landry said. “It really helped the channel define itself quickly.”

Prior to the removal, about 85 percent of the streambed upstream from the Maxwell Pond Dam was sand and silt — not particularly good habitat at that rate. Downstream, only about 5 percent of the streambed was sand and silt. That’s not good either, as certain species need at least some sand and small particles for spawning. Now the streambed is closer to a healthy equilibrium on both sides, Landry said. 

Landry said there is a massive delta of sediment where the Black Brook enters the Merrimack River. That delta is largely caused by the sediment from behind the dam. It may take a while, but that sediment will eventually work its way out into the river and downstream. “It’s not a bad thing, it’s a transitional thing,” Landry said.

While sediment is flowing downstream, there are some things catching biologists’ eyes upstream on the Souhegan River as well. The river is flowing substantially faster upstream of where the dam once stood. Faster flow equals more erosion. Snyder said it’s been interesting to see how the river sort of controls its own erosion. As the river cuts into banks, trees begin to fall and float sideways alongside the eroding bank, forming a natural barrier and slowing erosion. The tree sort of pushes the water and its erosive forces to the other side. That process of erosion is natural, but the removal is allowing Snyder to watch it happen on fast-forward.

“It really underscores how important big trees are to river ecology,” Snyder said, noting that six or so trees upstream have fallen in that fashion. “Erosion is causing erosion to stop. It’s not a process we’ve never seen before, but this is very dynamic. Normally you’ll have a tree fall in once in a while.”

One fish, two fish...
“When you remove a dam, you allow fish to move freely back and forth, in this case providing access from the Merrimack into the Souhegan,” said John Warner, fisheries biologist with New England Fish and Wildlife.

Warner said officials are expecting to see sea lamprey and American eels now access the Souhegan River. Previously, they would not have been able to. Dams in Lowell and Lawrence have different types of fish passage systems, which some species have harder times than others navigating.

Spawning fish, such as Atlantic salmon, have developed so they are born in freshwater and travel to the ocean where they spend most of their lives. The fish return to freshwater to spawn. Salmon, shad, river herring and sea lamprey all attempt to travel up the Merrimack River to do so. This year 78 salmon were collected at the dam in Lawrence. (The few wild salmon trying to get upstream are taken to hatcheries to maximize spawning potential.)

“The long-term goal is that we would stop having to have hatcheries,” Warner said. “The reality is that’s currently not realistic right now.”

Warner said the Souhegan River could be one of the first places with natural salmon runs. The American shad is another species that stands to benefit from the dam removal.

“What we’re seeing now is areas are now clear of sand and running free, more like what it would have been,” Warner said. “The young Atlantic salmon that were stocked upstream worked their way down. They move around, but they’ve found places that are good places to be. It’s exciting. It’s a direct impact of what was done a year ago.”

Biologists are seeing a shift in fish species on Black Brook as well. Downstream of the dam, biologists typically find 15 to 17 different fish species. That number hasn’t changed, but since the dam was removed, biologists found a tessellated darter for the first time, a key indicator for the stream’s health. Upstream of the dam, biologists actually didn’t find some of the warmwater fish they historically had found — an indicator the stream is cooling down into healthier stream habitat — and they found an American eel upstream for the first time.

“We were thrilled when we saw that,” Landry said.

Black Brook’s water probably stays too warm even without the dam to support fish like trout and salmon, Landry said.

Monitoring
The National Oceanic and Atmospheric Administration (NOAA) is especially interested in the monitoring of dam removals. They look at specific parameters so they’ll have consistent data to measure for removals going forward, said Matt Collins with NOAA.

A partnership of government agencies, organizations and researchers are looking at sediment, fish, bugs, the river bed, the geometry of the river, water quality and plants. Officials are also taking repeat photos of waterways at specific cross-sections.

“It’s to help better understand the project results,” Collins said. “And to help refine the techniques that we’re doing. But also to better communicate the results to the public.”

The monitoring protocols put scientists in a better position to answer questions about Black Brook and other water ways as they “devolve,” Landry said. It allows officials to track progress quantitatively.

“A lot was happening in the region without much monitoring,” Collins said, adding that officials faced criticism for not having documentation to back up the benefits they said would occur. “We just didn’t have a lot of data to actually document to what degree benefits were accruing.”

Now that agencies are spending time monitoring results they predicted to occur anyway, Collins said they face criticism for monitoring something they “knew” was going to happen.

“We’re not discovering anything that we didn’t largely predict,” Collins said. “What we are learning is an awful lot in the details of what happened.” As dam removal data builds, Collins said, agencies would likely have to do less monitoring in the future.