Small Boat, BIG Science!

They call it the Alaskan Milk Run.  "Combis," planes outfitted to haul cargo forward and passengers aft, jump between the burgeoning metropolis of Seattle and Anchorage, lingering at isolated air strips in Alaska’s southeastern panhandle long enough to deliver goods and humans to the small communities that dot the coastline. We (Sean Gulick, John Goff, Marcy Davis (UT), Dan Lawson (CRREL, whose study of the Hubbard is funded by NSF), and Bryce Willems (UAA)) caught the Seattle-Juneau-Yakutat-Cordova-Prince William-Anchorage run, thankful our destination was only a hop and a skip and that the views out the starboard side of the plane were those that make geo-dorks like us giddy with the anticipation of field work in one of the most rugged, wild terrains on earth, the St. Elias Mountains.

We reached an unusually sunshiny Yakutat about noon. A dusty outpost with awe-inspiring views of Mt. St. Elias, Yakutat serves up a quieter version of the Alaskan fishing vacation. With no cell phone service, rusting thirty-year-old rental vehicles, and few paved roads, the place feels like heaven to those hoping to escape the noisy lower 48.

Next, another plane—this time, a Cessna with just enough room for six, operated by Les Harltey, owner of Alsek Air Service.  During a glorious hour and half flight, Les took us up the Situk River to Russell Fiord and Hubbard Glacier, North America’s largest tidewater glacier—and the subject of our study—to recon ice conditions along the glacier front.

Unlike most glaciers, which have thinned and retreated during the last century, Hubbard Glacier is thickening and advancing. In the last twenty-five years, the Hubbard has twice advanced to close off the narrow gap between Russell Fiord and Disenchantment Bay, creating the largest glacier-dammed lake in North America, Russell Lake.

During both damming events, in 1986 and 2002, flow out of Russell Fiord slowed long enough to raise water levels and significantly decrease water salinity, threatening the fiord’s sea life. In 1986, water levels behind the dam rose 25 meters. In both cases, large floods rolled into Disenchantment Bay when the dam finally broke.

If Hubbard Glacier continues advancing, Russell Fiord could become dammed again. Although the last two dams broke within one season, they managed to raise water levels behind the dam to dangerous levels in a very short period of time, creating the largest glacial-lake outburst floods ever recorded. If another dam forms Russell Lake could overflow backwards and flood westward down the Situk River valley, threatening a vital fresh-water ecosystem and possibly submerging the Yakutat airport.

Following our flight, we headed to the U.S. Forest Service garage for a look at our boat, Quest, a thirty-five foot aluminum-hulled sport-fishing and tour boat owned and operated by Mark Sappington of the Yakutat Charter Boat Company. Our goal: rig her with geophysical instruments, sail her to the glacier front, and survey the sea floor between Disenchantment Bay and Russell Fiord. Oh, and get a few samples of what’s down there, too.

Our interest in the Hubbard lies in understanding factors that contribute to glacier advance despite a warming climate. We also want to know the potential hazards of ice dam formation and collapse to Yakutat and surrounding areas. To get at these questions, we used sound to get a look at and under the seafloor in the 300 meter gap between Hubbard Glacier and Gilbert Point. We also surveyed into Russell and Nunatak Fiords, hoping for glimpses of the faults which stretch between the St. Elias Mountains northward to Denali as they may have something to do with the Hubbard’s strange behavior.

The sidescan sonar is a forty-pound, four-foot-long, yellow, torpedo-shaped ‘fish’ which, when used in combination with other methods which directly sample the sea floor, can help determine the material and texture of the sea floor. As the instrument is towed behind the boat, it emits pulses of sound from its sides. A computer inside the boat’s cabin measures the intensity of the energy that bounces back from the seafloor and outputs images that look something like a black and white photograph. The darker the color, the more energy is being absorbed and so the material likely consists of soft or unconsolidated materials like clay. A lighter color indicates higher reflectivity—that the seafloor may be made of harder material like compacted sand or bedrock.

The Chirp is similar, but different. The instrument has a funny shape and weighs close to three times as much as the side-scan. It’s just awkward and heavy. All that weight is in her nose to help her fly right underwater as she’s towed behind the boat. Well, that’s the idea anyway. Despite the five of us being capable humans, we weren’t quite sure how we would get her in and out of the water. Eventually, with some welding, a drill, and a few hours, we managed to piece together a davit, a couple of pulleys, and a hand crank. Even so, it took three people to get the chirp in and out of the water. Good thing she earns her keep! The chirp emits a range of frequencies that penetrate the seafloor. The energy reflected back to the sensor provides an image of the stratigraphy and structure below the seafloor. And she gave us some good stuff.

Surveying with two towed instruments in the water along with a whole lot of ice (from the calving 100-meter-tall glacier) and tidal currents that whip through the gap at several knots was not as easy as it, er, sounds. Pretty exciting at times, actually. Vigilance was key, both from the standpoint of keeping bergy bits from getting caught in the data and tow cables, ice fouling the quality of the data itself, and from a purely navigational standpoint.

After four days of data collection, we’re feeling pretty good about the work we accomplished. It will take some time for us to understand everything we’ve seen and to interpret all our data, of course. And, yes, we really like our jobs. —Marcy Davis