South Cascade Glacier
By: Doug Geller
Alpine glaciers are relatively small bodies of ice that are relatively sensitive to changes in the patterns of temperature and precipitation. This makes them very useful indicators of climate change, which is inferred by measuring the annual budget for a glacier called its mass balance along with other climatic and hydrologic factors. The mass balance is simply the difference between accumulation (snowfall) and ablation (melting, and other processes such as ice-calving). While weather and hydrologic observations have been conducted for well over 100 years in many parts of the world, only relatively recently has glacier monitoring been recognized as useful not only for understanding climate effects but for forecasting hydrologic conditions in drainage basins dependent on summer runoff originating as snow and ice meltwater.
This paper examines the ongoing study of the South Cascade Glacier located in Washington's North Cascades. The glacier is so-named because it forms the headwaters of the South Fork of the Cascade River, but its location is within the North Cascades which is a distinct and very rugged mountain range in northern Washington and Southern British Columbia where there are hundreds of alpine glaciers. The South Cascade of one of three glaciers studied in detail by the U.S.G.S. - the other two are in Alaska. On an annual basis the glacier shows both positive and negative mass balances in response to precipitation and temperature but in the longer term, the glacier mass has declined more than 50 percent in the last 50 years and has retreated significantly up-valley as shown clearly in the three photos from 1928, 1979 and 2003.
Approximate glacier area in 1928 versus 1979: 1.49 km2 vs 0.49 km2
U.S.G.S. Glacier Monitoring Strategy
The U.S.G.S. glacier monitoring strategy is detailed in Fountain and others (1997) and is based on a hierarchy of monitoring techniques to be applied within each of nine identified glacierized regions in the U.S. These regions include, for example, the Front Range of Colorado, Wind River Range in Wyoming,Glacier National Park in Montana, the Alaska Range and other ranges in Alaska, and the Cascades and Olympics in Washington and Oregon. The purpose is to establish detailed understanding of glacier mass-balance (i.e. dynamics of accumulation and ablation) and response to climatic factors.
The monitoring strategy is founded on a 'nested approach' whereby one glacier is selected as the Benchmark Glacier and is studied intensively throughout the seasons (see Methods below), and a Secondary subset of other glaciers are studied less intensively with annual measurements made, and a third grouping of glaciers which are monitored using only indirect (remote sensing) methods.
The USGS program to date monitors only three glaciers using detailed field measurements. However, two complementary programs combine with the South Cascade Glacier monitoring to make the North Cascades an area with one of the most extensive glacier mass balance monitoring systems in the world.
The North Cascades Glacier Climate Project (NCGCP; Pelto and Riedel 2001) has monitored 8 North Cascade glaciers since 1984 and added two more in 1993-94. The National Park Service (NPS; Pelto and Riedel) has monitored four glaciers since 1993-94. The glaciers monitored comprise a range of alpine glacial settings, including varying aspects and elevations.
South Cascade Glacier Setting
The South Cascade Glacier is a small valley glacier near the Cascade crest in the section known as the North Cascades of Washington. The glacier lies at the head of the South Fork of the Cascade River, which is a tributary of the Skagit River which drains the western part of the Cascade Range and empties into Puget Sound in between Seattle and Bellingham, Washington. Local relief exceeds 3,000 feet with high peaks exceeding 9,000 feet in elevation. The North Cascades are characterized by a maritime climate, with very high precipitation, with amounts reaching 15 feet annually (Meier and others 1971). From October to April, most of the precipitation in the alpine zone of the range falls in the form of snow. Individual storm totals can often exceed two feet.
The USGS website provides links to several ongoing studies related to the South Cascade Glacier and the glacier monitoring program.
Glacier Monitoring Methods
The South Cascade Glacier is one of three Benchmark Glaciers that have been established in the monitoring program. Benchmark Glaciers are studied with permanent field monitoring stations which measure precipitation, snow depth, snow density, humidity, surface movement, surface altitude, and basin runoff throughout the season. Besides the South Cascade Glacier, the U.S.G.S. intensively monitors the Gulkana Glacier in the Alaska Range and the Wolverine Glacier in the Chugach Range of southern Alaska, with plans to add other glaciers in the future as funding allows. Within the region of each of the Benchmark Glaciers, annual field measurements are made at several other glaciers used as secondary monitoring sites and finally a larger number of glaciers are observed with remote sensing images. The remote sensing is particularly useful in assessing the ratio of the accumulation area to the total glacier area as a check on trends in glacier mass balance in each monitored region.
The National Park Service (NPS) and the North Cascades Glacier Climate Project (Pelto and Riedel 2001) conduct annual balance monitoring of 14 additional glaciers. Typical physical field measurement methods include the use of ablation stakes which measure snow depth across the glacier through a complete melting season, snow/ice surface altitude, and measurments of crevasse positions and dimensions. Glacier Monitoring Website provides a good summary of monitoring that shows the regional correlation between glacier balance and regional climate.
Graphs showing cumulative mass balance trends for three Benchmark glaciers
Annual Mass Balance Trends
The 1970s through the mid 1990s was a period of generally negative annual mass balance, which resulted in reductions in ice volume and glacier surface area, and was characterized by up-valley glacial retreat. The retreat of some glaciers has exposed several 'new' small glacial lakes or tarns. An example is the lake now found below the terminus of the South Cascade Glacier (1979 and 2003 photos). The strong correlation in mass balance trends between different glaciers in the North Cascades has led researchers to conclude that the monitored glaciers are responding to regional scale climate factors and not local scale conditions or microclimatic factors. Heavy snow accumulation years in the late 1990s (in particular, 1998-1999) resulted in positive annual balances, which slowed the general trend of glacier shrinking in the North Cascades. The data are not yet available from the last 2-3 years which have been highly variable including the very heavy snow year in 2005-2006. The above graphs showing the cumulative balance of the three Benchmark glaciers suggest that the overall trend is negative over the past several years. It is interesting to note that the South Cascade Glacier has been retreating faster than other glaciers in the North Cascades monitoring system, and even though it is the benchmark glacier it may not be entirely representative of the 'health' of glaciers in the range.
Glacier mass balance monitoring provides valuable hydrologic and climate data particularly in areas where glaciers are sensitive to short and long term climate change. The data are also particularly useful in helping to make accurate predictions of the hydrologic impacts of climate-driven changes in glacier mass and annual meltwater production.
The South Cascade Glacier has been intensively monitored and studied for nearly 50 years and in the past decade, a number of other nearby glaciers in the North Cascades have been monitored somewhat less intensively to provide a regional framework for evaluating glacier response to climatic factors.
The regional monitoring network in the North Cascades has shown that the glaciers respond mainly to large scale climatic factors and not microclimates. There is strong regional accordance in glacier response to climate, but there are variations. The retreat of the South Cascade Glacier has been noted as an indicator of climate change ('global warming') but it is interesting to note that its mass balance has generally been more negative than the other glaciers in the monitoring system. Still, there is a general trend of shrinking glaciers in the North Cascades that continues to pose a concern.
Fountain, A., R. Krimmel and D. Trabant, 1997. A Strategy for Monitoring Glaciers. U.S.G.S. Circular 1132.
Krimmel, R. 2001. Water, Ice, Meteorological and Speed Measurements at South Cascade Glacier, 1999 Water Year. U.S.G.S. Water Resources Investigation Report 00-4265.
Pelto, M. and J. Riedel 2001. Spatial and Temporal Variations in Annual Balance of North Cascade Glaciers, Washington 1984-2000 North Cascade Glacier Climate Project,Hydrologic Processes 15, 3461-3472.