An Integrated Approach for Monitoring Changes at Bering Glacier, Alaska
Bruce F. Molnia, U.S. Geological Survey, Reston, VA 22092
Austin Post, U.S. Geological Survey (Retired), Vashon, WA. 98070
Monitoring changes at Bering Glacier, Alaska has been accomplished through a multi- faceted approach involving the collection and analysis of a variety of remotely-sensed data, field measurements and observations, and the analysis of historical maps and reports, leading to the development of a geographic information system (GIS). Integrating the results from these individual approaches has produced a detailed understanding of the twentieth century history of Bering Glacier and a comprehensive picture of the 1993-95 surge of the glacier.
Bering Glacier, located in coastal southcentral Alaska, is the largest and longest glacier in continental North America (Molnia and Post, 1995). During the twentieth century, as much as 12 km of terminus retreat, accompanied by several hundred meters of glacier thinning, and as many as six major surges have occurred. Documenting this complex sequence of events and monitoring the resulting changes has been possible through a multi-faceted approach involving the collection and analysis of a variety of remotely-sensed data; field measurements and observations; and the analysis of historical maps and reports, some more than 150 years old. Much of these data are being incorporated into a developing geographic information system (GIS).
Monitoring changes at Bering Glacier, or for that matter any glacier, requires establishment of a baseline(s) or other reference standard(s). Later observations are then compared to this baseline(s) or standard(s), resulting in the determination of quantity and rate of change. For Bering Glacier, this baseline is complex and dynamic, consisting of multiple baseline data sets.
For example, the position and elevation of the glacier's terminus was known from field observations and mapping since the first half-decade of the twentieth century. Additionally, much information about the glacier's geography was previously known from the nineteenth century (Molnia and Post, 1995). These observations provide a baseline against which to measure twentieth century retreat and thinning. Later observations have provided details about changes in terminus position in the early 1920s, late 1930s, late 1940s, early and late 1950s, and virtually every year since 1960. Since August 1993, the start of terminus displacement in the latest surge of Bering Glacier, field and remote-sensing observations have been made almost monthly.
One of the first steps necessary to monitor and record changes at Bering Glacier was the compilation and construction of a complete map base of the Bering's eastern piedmont lobe. The base map is being used for depicting the many identified locations of the glacier's terminus since 1900 and for plotting the locations and values of measured parameters. The map is based on U.S. Geological Survey topographic data (1904 - 1906, 1957, 1972), GPS measurements (1990 - 1995), and vertical aerial photography (1990 - 1995).
This map presently is the basis for "analogue" or paper monitoring and change detection. This map is also being used as the base map for the development of a digital Bering Glacier Geographic Information System (GIS) currently being developed jointly with the U.S. Bureau of Land Management (BLM), an agency of the U.S. Department of the Interior. The GIS is being formulated for the integration, display, manipulation, and storage of the various geographically located data and information available for Bering Glacier.
During the 1993 - 1995 surge, airborne remotely-sensed data were collected approximately every 30 - 60 days. Combined with the quantity and variety of all of the other data collected during the surge, this may be the best documented surge event of any glacier.
Bering Glacier Data
Airborne and spaceborne remotely-sensed data of Bering Glacier include:
Field observations and measurements include:
Historical data include:
Description of Bering Glacier Data
Aerial Photography -- More than 75 vertical and oblique aerial photographic data sets, containing more than ten thousand aerial photographs of Bering Glacier have been collected. These sets date from 1938 to May 1996 and include images ranging in size from 9 in by 9 in to 35 mm. As the USGS, using differential GPS, has precisely determined the location of more than 100 geographic features within the Bering Glacier study area, photographs containing these features, especially those collected vertically, can be geographically referenced and used for precise change measurement and for inclusion in the Bering Glacier GIS.
The earliest aerial photographs, made by Bradford Washburn in 1938, while exploring mountain climbing routes in the Chugach and Saint Elias Mountains, clearly depict terminus position, outwash plain configuration and location, marginal lake size and level, and recent surge history. Later aerial photographs provide data to continue to monitor changes in these parameters. Since about 1960, vertical and oblique photography has been collected almost annually by the USGS. They have been the source of much detailed information for continuation of monitoring changes.
Since May 1993, more than 20 oblique color and black-and-white aerial photographic sets and more than 10 vertical aerial photographic sets have been collected to document surge related changes.
Landsat Imagery -- more than 50 multispectral scanner (MSS) and thematic mapper (TM) Landsat images dating from 1972 - 1992 have been obtained. As each of these images covers about a 200 km by 200 km area, virtually all of Bering Glacier can be observed at many single points in time. These images are being used in both an analogue and digital manner to monitor changes in terminus position, changes in snow cover and ablation, sediment production and dispersal, surge displacement of medial moraine features, and changes in proglacial vegetation. As all of these Landsat images are geographically registered, they will be a vital component in the information base of the Bering Glacier GIS.
Spaceborne Synthetic Aperture Radar Data (SAR) -- Since 1992, about 50 digital satellite SAR images of Bering Glacier have been collected by the SAR sensor on the European Space Agency's ERS-1 satellite. These images, downloaded at the University of Alaska's SAR Facility (ASF) in Fairbanks, have been processed by USGS to monitor and determine rate and style of ice displacement during the 1993-94 phase of the latest surge, changes in terminus position, changes in snow cover and ablation, and sediment production and dispersal. Bering Glacier was selected by the National Aeronautics and Space Agency (NASA) to be an ecological research site for 1994 and 1995 environmental missions of the Shuttle Imaging Radar (SIR-C)experiment. As a result, two additional radar data sets of parts of the terminus were obtained. Digital SAR data can be merged with other digital remotely sensed data and incorporated into the Bering Glacier GIS. Mapped derivative information can be scanned into the GIS as well.
Airborne Synthetic Aperature Radar Data --Two other side looking, airborne radar images (SLAR), high-resolution X-Band, SAR radar images, of Bering Glacier were collected by the USGS in 1986 and 1990. They have been used to monitor changes in terminus position, changes in snow cover and ablation, sediment production and dispersal, and iceberg production.
Airborne video -- about 15 hours of airborne video has been collected between 1989 and May 1996. This video, collected in VHS, S-VHS, and 8 mm formats, shows numerous surface features of the glacier from before, during, and after the latest surge. These data are useful as a supplement to the numerous photographic data sets.
Feature mapping -- Since 1974, field studies have been conducted to "ground truth" many geomorphic and geographic features that had been identified from aerial photography. This has included confirming the identification of and age of moraines, and the character of surface and outcrop sediment deposits.
Time lapse photography -- Time lapse photography from in-situ camera systems has been used since 1993 to monitor changes during the latest surge of the glacier. In 1994, as many as five cameras were deployed simultaneously. Cameras ranged from a single 16 mm movie camera which took one image every five minutes to several 35 mm cameras which took one image every 24 hours. The 16 mm film has been converted to video.
Sequential photography -- reoccupation of marked photo stations has yielded many pairs, triplets, and multiple-year sequential photo sets which are excellent for monitoring change. In 1995, the authors relocated and revisited photo stations originally established in 1946 by USGS geologist Don J. Miller. Comparison of the 1995 photographs with the 1946 photographs document 50 years of change at many ice-marginal locations north of the piedmont lobe.
Discharge measurements and stage information from telemetering recorders -- In 1991 and 1993, USGS hydrologists determined the discharge of the Seal River, the largest distributary of the Bering Glacier. In both instances, discharge was calculated using the cross-sectional area method. Beginning in 1994, recording stage gauges have been used to monitor surge related discharge fluctuations, with the data telemetered to Fairbanks, AK. Several flood events associated with the surge have been identified.
Ice-penetrating radar surveys -- In 1990 and again in 1993, USGS collected a total of more than 100 ice thickness measurements using ice-penetrating radar. These surveys have provided detailed information about the depth to the bedrock underlying the glacier and the thickness of the ice at the time of the survey. The location of each survey point was determined using GPS. Knowing the elevation of the bed permits calculation of changes in glacier thickness through time. This has permitted monitoring of total thickness changes during the period 1972 - 1991, a period of time in which the glacier thinned as much as 165 m (20%) (Molnia and Post, 1995).
High resolution marine seismic reflection surveys -- high resolution marine seismic reflection surveys of Vitus Lake, the glacier's ice marginal basin were conducted in 1991 and 1993 using GPS navigation. These surveys, which yielded profiles of more than 500 km of the basin, showed that much of the basin is more than 200 m below sea level and is filled with as much as 110 m of recent sediment. This information, combined with a knowledge of ice margin locations and the date of and sequence of glacier retreat permits detailed monitoring of sedimentation rates and volumes. Since much of the area that was profiled was subsequently covered by advancing ice in the 1993-95 surge, a baseline of pre-surge depth and sediment thickness information now exists for comparison with post-surge retreat measurements.
Seismic refraction surveys of the glacier's outwash plain -- Seismic refraction surveys of the glacier's outwash plain were performed from 1991 to 1993. These data determined the depth to bedrock underlying the glacier's forelands and the thickness of sediment. Based on these measurements, it was determined that the forelands overlie a deep basin which extends several hundred meters below sea level and which connects to a deep submarine trough in the Gulf of Alaska.
Dendrochronological, paleobotanical, and tree coring studies -- Studies conducted since 1976 have resulted in 32 14C dates obtained from peat, sub-fossil wood, and shell samples collected by the authors from the margins of the glacier. Additionally, a 1,200 year tree ring chronology, with "floating" calendar dates for some samples reaching nearly 2,000 years, augment the other glacier data. This information has resulted in dating of many exposed moraines. Additionally, monitoring of recently formed moraines permits an understanding of rates of and types of vegetation development. This information is useful for determining the sequence and age of many twentieth century moraines around the margin of Bering glacier.
Monitoring of movement and erosion stakes at selected terminus sites -- Beginning in 1993, lines of stakes were set out at about 20 locations around the surging margin of the glacier. These lines were checked on a daily to weekly cycle to monitor short term rates of terminus change. Results showed that the western margin of the glacier advanced more rapidly than the eastern and that maximum daily rates of advance on land were about 5 m per day. In July 1995, one group of stations was placed to monitor the amount and rate of shoreline bluff erosion and retreat associated with the advance of the glacier's southeast margin. Interpretation of the resulting measurements suggest bluff retreat rates approaching 1.5 m per day.
Precision determination of feature locations using differential GPS -- Between 1992 and 1995, more than 100 geographic features identifiable on USGS vertical photography were precisely located using differential GPS. Exact positioning of these locations permits detailed calculation of changes in many different types of features. Additionally, each of these features can be incorporated into the GIS base map.
Water sampling - Between 1976 and 1980, and again in 1992, 1994, and 1995, water samples were collected from Vitus Lake and the Seal River. These samples were used to monitor changes in the suspended sediment load of the lake and river and to determine the temperature and salinity of the lake. Data collected in 1992 indicated that the lake was actually a marine embayment with near-marine salinities. The subsequent surge flushed nearly all of this marine water from the basin and changed salinities to those of nearly fresh water.
Analysis of eighteenth, nineteenth and early-twentieth century exploration maps, description of voyages, and reports of expeditions, nineteenth century published maps and nautical charts, and late nineteenth and early twentieth century geological reports has provided a much longer term perspective on the glacier's post-Little Ice Age history than could be obtained from present day measurements. For instance, a description of the character of the glaciers surface from the late 1830s by Belcher (1843) suggests that the glacier was surging at the time of his observation. Similarly, field photography obtained by an 1897 climbing expedition and a 1905 geological field party clearly show the character and position of parts of the glacier's margin. All of these historical data are useful for understanding how the glacier has behaved in the past.
A variety of techniques have been used to monitoring changes at Bering Glacier, Alaska. Monitoring has been accomplished through a multi-faceted approach involving the collection and analysis of remotely-sensed data, field measurements and observations, and the analysis of historical maps and reports. Ultimately, these data will be incorporated into developing a geographic information system (GIS). Integrating the results from these individual approaches has produced a detailed understanding of the twentieth century history of Bering Glacier and a comprehensive picture of the 1993-95 surge of the glacier.
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Belcher, E.B., 1843, Narrative of a Voyage Around the World Performed in Her Majesty's Ship Sulphur During the Years 1836-1842. London; Henry Colburn, Publisher.
Molnia, B.F. and Austin Post, 1995, Holocene History of the Bering Glacier, Alaska: A Prelude to the 1993-1994 Surge. Phys. Geogr., 16, 87-117.