Surfer Used to Find Headwaters of the Yukon River

The Yukon River in Alaska and northwest Canada, at 2,000 miles long, is one of the longest rivers in the world. Although it empties into the Bering Sea on the west coast of Alaska, its ultimate source is a glacial divide on the Juneau Icefield in southeast Alaska. Snowmelt on the south side of the divide travels to the sea in a distance of only 40 miles, while snowmelt on the north side of the divide must flow 2,000 miles through the Yukon River before it reaches the sea.

The Juneau Icefield Research Program has been conducting glaciological, meteorological, and geological research on the Juneau Icefield every summer since 1946. One of the objectives of the program is to monitor changes in the glaciers of the Icefield in an attempt to understand the effects of global warm movement and surface elevation surveys using highly accurate real-time GPS. At the same time, seismic refraction studies are carried out along the survey profiles to determine the depth of the ice and the bedrock topography.

In order to relate physical changes of the glaciers to climate change, deep ice cores are typically drilled and analyzed. These cores can contain a record of past climate stretching back hundreds or thousands of years. In 1995 the Juneau Icefield Research Program (JIRP) initiated a program to investigate the possibility of retrieving an ice core at the divide between the Matthes and Llewellyn Glaciers on the Juneau Icefield. This divide also happens to be the true headwaters of the Yukon River, although the exact point at which the hydrographic flow divide occurs is not evident due to the fact that the divide is actually a nearly flat, featureless plain of ice and snow. One of the requirements for obtaining an ice core in this area was that it be extracted from the area of least glacier movement, thus ensuring a core that had undergone the least amount of deformation.

In the summer of 1997 a grid pattern of 30 survey stakes was established at the divide. These stakes were then surveyed with high precision GPS to obtain surface movement and surface elevation data. Seismic refraction and gravity readings were then taken at each flag to determine the depth of the ice at each flag. The GPS survey data and the geophysical data were then used in Surfer to create grids of the glacier surface topography, surface movement, and bedrock topography. Using Surfer, a three-dimensional plot of the surface and bedrock topography was generated. The plot also included surface movement vectors. In order to determine the exact location of the minimum surface movement, and hence the optimum location for drilling an ice core, the surface movement grid used the daily surface movement at each flag as the Z-value. This grid was then converted to an XYZ data file, the Z-values were sorted in a Surfer worksheet in ascending order, and the exact X and Y locations of the minimum surface movement were easily found.

In evaluating the three-dimensional Surfer plots, we have found that, unexpectedly, the location of the surface glacial divide does not coincide with the bedrock topographic divide; the bedrock divide being located approximately 711 meters northwest of the surface divide. We also found that the movement vectors of the ice are not the same on the glacier surface as they are at depth. Most significantly, the use of Surfer for this project has allowed us to determine, in a cost effective manner, the optimal site for a future ice core drilling project on the Juneau Icefield. And as an added bonus, we have for the first time located the exact point of the beginning of the Yukon River.

-Scott McGee, JIRP Surveyor

Surface and Bedrock Morphology of the Matthes/Llewellyn Divide.

Upper surface shows the surface topography and movement vectors in the area of the Matthes Glacier-Llewellyn divide. Arrows indicate the magnitude and direction of the surface movement at each survey flag. This minimum movement is between flags 29 and 30. Vectors and elevations were obtained via real-time differential GPS.

Lower surface shows the subglacial valley at the Matthes-Llewellyn divide, with the actual bedrock divide being located between Flags 14 and 18. Subglacial topography was determined by using a combination of seismic and gravimetric methods. The maximum observed ice thickness was 992 meters at Flag 20, while the minimum was 442 meters at Flag 4. Note that the area of minimum surface movement is not coincident with the bedrock divide.

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