Missisquoi National Wildlife Refuge in Vermont had a complicated problem this past winter, and a good man for the job came up with an ingenious solution to it—an ice bridge.
The primary problem was that a Native American cultural site containing artifacts from the Abenaki tribe and others needed protection from Missisquoi River erosion. Erosion was washing archaeological history downstream. In 2010 alone, 20 feet of bank eroded along the site. Something had to be done.
Natural Resources Conservation Service engineers—with approval from U.S. Fish and Wildlife Service engineers, the Abenaki tribe and the Vermont Division for Historic Preservation—developed a protective design using rip–rap rock.
The refuge was able to acquire rock to begin the project last fall, but it was on the wrong side of the river. A refuge barge normally used to move materials on the river was inoperable because August 2011 flooding had marooned it in silt. Presented with this secondary problem, I, as refuge manager, figured that we’d have to wait for a permit to free the barge before moving any rock.
However, Joe Bertrand—a native Vermonter, an Abenaki tribesman and an equipment operator at the refuge for the past 10 years—had a different idea: Under proper conditions, we could move the rock quicker and cheaper to the other side of the river via an ice bridge.
Having no ice bridge experience, I naturally was reluctant to approve driving dump trucks full of tons of rock over a frozen river. I soon learned, though, that the science of ice loading is advanced. Leonard Zabilansky of the U.S. Army Corps of Engineers Cold Regions Research Engineering Laboratory in New Hampshire said air/ice temperature, ice thickness and ice sheet buoyancy over the river water were the key safety factors.
Timing was everything. The ice needed to be built up quickly to the necessary thickness. So, from early January to mid–February, Bertrand spent a dozen cold afternoons/evenings diligently pumping river water onto the waterway’s frozen surface at a selected place to increase ice thickness thin layer by thin layer. It had to be 20 degrees or colder for the new layer to freeze properly, he learned. One evening, it was five–below zero.
Regional Support
Eventually, Bertrand built up an average of 16 inches of ice across the section, enough to allow us to move 15 tons across at a time if all other conditions were met. Fifteen tons is a lot, but we had 1,500 tons to move. One equipment operator could not do it, so we called for regional support.
Bill Starke, the Service Northeast Region heavy equipment coordinator, secured funds to rent two dump trucks and an excavator to load rock. He also arranged for equipment operators Stephen Zadroga of Eastern Massachusetts Refuge Complex and Kirk Cote of Aroostook Refuge in Maine to help Bertrand.
On February 13 and 14, as refuge biologist Judy Sefchick–Edwards and I monitored ice temperature, buoyancy and load conditions, Bertrand, Zadroga and Cote hauled rock across our ice bridge. “We didn’t mess around,” said Bertrand. “We worked until 11:30 that first night,” after starting at 8 a.m. They moved about 900 tons (60 truckloads) in two days before the weather warmed to an unsafe level.
“They did an awesome job,” Bertrand said of Zadroga, Cote and the regional support. “It never would have happened without them.”
The project won’t be completed for a few more years because there are at least 600 tons more rock to move and funding is not expected until 2013.
Nonetheless, it satisfies Bertrand to play a role in conserving a cultural site that holds documented artifacts—pottery sherds, whetstones, chert tools and arrowheads—from a settlement his forebears inhabited some 1,500 years ago.
“I’ve hunted it and fished the Missisquoi my whole life,” he said. “Some of my earliest memories are on the refuge.”
Ken Sturm is manager at Missisquoi National Wildlife Refuge.
