Environment
3:28 pm
Thu April 25, 2013

Stabilizing The Suncook: Pacifying A River Run Amok

Since 2006 the Suncook River has been on a different course: it jumped its bank in the Mother’s Day flood, and the state has been trying to stabilize it ever since. Now as part of a recent fine for filling wetlands, a gravel company will give the project 8,000 tons of stone for the project. But this is only part of a continuing effort to live next to a changing river.

Taken soon after the avulsion, this photo captures the cross-section of the ridge that used to contain the river, but which was overrun in the Mother's Day flood
Credit Eric Orff / Courtesy Photo

Before 2006, there wasn’t much to say about the Suncook. It was just a medium sized river, that running through Pittsfield, Epsom, and Allenstown to its confluence with the Merrimack.

Now, parts of it are a little hard to describe.

“My brain has trouble accepting the scale, of the distance, and just the vast…” DES watershed Manager Steve Landry’s voice peters out as he give up trying to explain what the erosion along the Suncook looks like.

Nearly 7 years ago a massive rain storm dumped nearly 15 inches of rain on parts of the state on Mother’s Day. That rain triggered a rare event, called an “avulsion” in Epsom. The river overflowed its banks, and for nearly a mile, carved itself a new path, cutting through the flood plain and carrying massive quantities of sand with it.

Wildlife biologist Eric Orff remembers the day well. He lives downstream and says huge chunks of the river were still falling in the next day when we took a trip to the spot where the avulsion occurred.

“About a mile and a half of the Suncook River simply disappeared during a one hundred year flood,” Orff explains.

One Flood, Long-Lasting Impacts

Today, visiting the old course of the Suncook, unless you have a trained eye, it’s hard to tell that a river used to run here: trees and grass have sprouted up.

But walk to the edge of where the river now runs, and the contrast is stark. The old course is mayb

The old course of the Suncook no longer looks much like a river bottom.
Credit Sam Evans-Brown / NHPR

  e forty feet across, but the new river bed is massive. “Now we’re looking several hundred feet across to the east bank of the Suncook River as it’s shifting through the sands,” says Landry, surveying the scene.

When the river took its shortcut, it was suddenly dropping the same amount of elevation in a much shorter distance. So the water sped up, and faster water carries more sand and clay particles with it.

If you visit the river now, the effects are obvious: the channel has dropped more than ten feet, as the faster water has eaten away at the sandy river bottom, and there are places were the banks continue to slide into the river.

All that sand has to go somewhere. According to US Geological Survey scientist Robert Flynn, it’s all downstream, in Allenstown. “Because of the movement of the sediment downstream, that’s increased the frequency of overbank flooding,” Flynn explains. And those living the in by the river have certainly noticed that increased flooding.

There are 110 homes in the floodplain downstream of the avulsion. $6 million in state and federal dollars have been spent to get those residents out of harm’s way. So far, the state and FEMA have purchased 35 homes and 18 more are waiting for additional funds, so that they too can find new – higher and drier – homes.

Still More Work Needed

But there are more concerns upstream, Landry says one spot is the bridge where the river runs under Route 4. “The bridge footings, they’re sitting on sand, basically,” he explains.

Steve Landry surveys what has come to be called the "avulsion riffle" - a short rapid just upstream of where the river jumped its bank. As the river works to find an easier way over the rocks here, it is eroding the far bank, toppling trees and taking property from landowners.
Credit Sam Evans-Brown / NHPR

After the river’s level dropped, those bridge footings aren’t rooted deep enough, and since the entire Suncook River valley is almost entirely sand, there’s nothing to stop the river from moving again in another flood. That could trigger another chain reaction of erosion, and undermine the bridge.

“With unlimited funds we would probably replace that bridge, and build a valley spanning bridge so the river can migrate where it wants to,” says Nick Nelson a river scientist with Interfluve, the consulting firm working with the state to stabilize the Suncook.

But that would cost the state as much as $15 million, and the bridge is relatively new. With many red-listed bridges around the state, replacing bridges still in good-shape is not in the cards. By contrast, it would only cost around $3 million to line the river bank with big stones that can’t be washed away. And that plan just got a boost, after Torromeo industries was ordered to donate $330,000 dollars’ worth of stone to the state for a wetlands violation.

Unfortunately, it’s a solution that isn’t particularly good for the river habitat. “It’s a compromise, really, that it’s not a typical river restoration project, period. It’s a stabilization project,” explains Landry.

While rivers are constantly moving, it’s usually on a geological time scale: over many, many years. After the avulsion was like seeing science in motion as the river at times violently found its new course. Once the state gets together the rest of the money and materials it needs – which might take a few more years – the hope is, this river will again be frozen in place, for a good long while.

Once the site of a lazy meander in a slack river, the site of the avulsion is now a scene of massive erosion and change.
Credit Sam Evans-Brown / NHPR