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Mount Baker, Washington. From the USGS Cascades Volcano Observatory webpage.
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Introduction
In order to understand the spatial variation of past and present alpine glaciers, it is essential to examine the factors that control their distribution. Because glaciers are intimately linked to the snowline, the controlling factors of the snowline’s patterns and behaviors can generally also be thought of as controls of the distribution of glaciers. While these controlling factors vary in their rank of importance depending on scale, there are several major factors that can be identified as the most critical over all. These factors are temperature, precipitation, altitude, latitude, continentality, aspect, and relief. This paper will begin with an explanation of the different meanings given to the term "snowline," followed by an examination of these controlling factors. Slope exposure as a controlling factor will be emphasized by a study using GIS. This study was used to illustrate what an important influence aspect has on the elevation of the snowline. The region used for this study is a portion of the North Cascade, Washington.
The influences of aspect on the snowline, glacier orientation, and glacial erosion have been known for many years. Gilbert (1904) described the asymmetry of crest lines in the High Sierra Nevada of California. In an examination of the range, he noticed that cirque erosion was exaggerated on the northeast faces of the peaks. Graf (1976) examined the location of cirques in the Rocky Mountains of Colorado, and found that the glaciers of past climates had had a preference for northeast facing cirques, provided favorable cirque morphology and adjacent topography. Allan (1998) used a GIS to analyze the topoclimatic influences on equilibrium line altitudes of modern glaciers in Glacier National Park, Montana. The influencing variables considered in the analysis included elevation, slope angle, slope aspect, shelteredness, and the solar radiation potential.
The snowline and glaciation limit have also been topics of interest to a number of researchers. Flint (1971) discussed the present snowline and spatial distribution of present glaciers so as to understand these distributions during former climates. Ostrem (1966) defined the term "glaciation level" and the techniques used to measure it, as well as clarifying the various classifications of the snowline.
Research has been conducted on the North Cascades and Mount Baker (Harper, 1993; Pelto, 1996; Post, 1971; Tangborne et al., 1990) to determine past fluctuations and the current trends advance or retreat.
Snowline
The term snowline is used to mean different things depending on the context in which it is used. In order to clearly present the factors that influence the snowline, the different meanings of the snowline must first be clarified. The term "transient" snowline is referred to as the boundary between a snow-covered surface and a bare surface at any given time (Ostrem, 1974). The transient snowline may reside on the ground as well as on the surface of a glacier. It is continuously changing, being near sea level during winter in some locations, or on the highest mountain slopes in the summer. The final height of the transient snowline at the end of the melting season will vary from year to year depending on changes in precipitation and temperatures. The term "climatic" snowline is given to the average elevation of the highest transient snowline on bare ground for a number of years. The "annual" snowline is the line or zone on a glacier that separates the snow-covered upstream part or accumulation zone from the downstream or ablation zone (Flint, 1971). It is measured at the end of the melting season, and so is a zone below which the previous winter’s snow was melted the previous summer.
The annual snowline is generally synonymous with the terms equilibrium line and firn line, in that it is also the zone at which the glacier mass-balance equals zero. At this point on the glacier, gains adding to the glacier’s mass equal losses from its mass, or accumulation is equal to ablation (Ostrem, 1974). Where the annual snowline continues on the ground between glacier the term "orographic" snowline is used. Local variations in the altitude of the orographic snowline are controlled by local topography and orientation. Finally, the "regional" snowline is defined as a large-area, or regional view of the orographic snowline, where it can be generalized into a band that varies in width from place to place, reflecting temperature and precipitation (Flint, 1971). The regional snowline is an important concept for understanding the distribution of alpine glaciers. It is also known as the glaciation level (Ostrem, 1974, 1966) The significance of this level is that it is the critical summit elevation which is necessary to produce a glacier on a mountain. The glaciation level may be calculated from a topographic map by determining a critical height somewhere between the lowest mountain in an area with a glacier and the highest mountain without one. On both mountains, the topographic conditions must be favorable. Some peaks have precipitous slopes, which do not allow for the accumulation of snow required for glacier formation, and so must not be considered.
Precipitation and temperature
Because precipitation and temperature are so closely linked to each other, it is essential to examine them together in order to determine their relationship to snowlines. In order for precipitation to influence the snowline there are two requirements. First, the precipitation must be in the form of snow. Rainfall does not contribute to glacier mass (Ostrem et al. 1967), nor does it have any significant impact on the snowline. The second requirement is that the net annual precipitation must be positive. If temperatures are too high, ablation will be greater than accumulation, which will result in a high or absent climatic snowline, and glaciers cannot be generated, nor can they advance. Tricart (1970) devised an index of snowfall effectiveness, the nivometric coefficient. This is calculated as the ratio of precipitation in the form of snow to the total annual precipitation in any form. A coefficient of 1.0 implies that the precipitation is entirely of snow. Regions of high precipitation and a nivometric coefficient that approaches 1.0 increase the likelihood of glacier genesis and sustainability. If the precipitation is low, but the nivometric coefficient is near one, the growth of glaciers is not encouraged, but long-term survivability is likely, as is the case of the Antarctic ice sheet. However, precipitation must not be considered as independent of temperature. Glaciers are generally found at temperatures that are below freezing. If summer temperatures are high, ablation may be too great to support glaciers (Sugden and John, 1976).
Latitude
Latitude has an indirect influence on the snowline in that it is a major control of temperature and precipitation. Its influence is less important on a local scale, but can be clearly seen on a global scale. The climatic snowline and glaciation level is highest in the tropics, while it is virtually the same elevation as sea level at the poles (Figure 1). Latitude is a limiting factor in the amount of solar radiation that reaches the earth in a given place. In the tropics, the angle of the sun is at or near directly overhead at noon. Moving away from the equator, the sun angle gets smaller and smaller, until it is at or below the polar circle, depending on time of year. In the Antarctic, prolonged winters and sub-zero temperatures are the perfect conditions for glacier formation.
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Precipitation decreases towards the poles, which are
essentially arid environments. Because of the frigid conditions, even though
precipitation is scarce, it falls in the form of snow, and mass balance
is always positive. Melting in this climate is insignificant. (Flint, 1971;
Ostrem, 1974; Sugden and John, 1976).
Figure 1: From Sugden and John, 1967.
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Altitude
Altitude can be seen as an independent variable at the local, as well as regional scale. Like latitude, altitude is a fundamental control of both temperature and precipitation. Temperature decreases with altitude, while precipitation increases up to a certain point, at least on the windward face. Temperature on a mountain decreases with altitude because of the environmental lapse rate, or the general cooling of the atmosphere as it looses its ability to store heat, and departs from the earth’s surface. A mountain does influence the temperature of the surrounding air, because of its ability to radiate heat. However, there is still a cooling trend with an increase in elevation. Precipitation increases up the slopes of a mountain due to the orographic effect, a situation involving the interplay between a mountain barrier and moisture-laden winds. Winds encountering a mountain barrier are forced to rise. In doing so, the air cools. Cool air can hold less moisture than warm air, and so condensation, which leads to cloud formation, is the result. If a mountain is high enough to penetrate this cloud layer, the zone of maximum precipitation is reached. Above this point, precipitation decreases, and the result is a high alpine desert. The interplay of altitude, latitude, and the climatic snowline can be seen in the tropics, where glaciers only are found at the highest altitudes (Evans, 1969; Flint, 1971; Price 1981., Sugden and John 1976).
Relief and topography
Alpine glaciers are found only where snow has room to accumulate to a large enough mass. Because of this, glaciers have a preference for peaks that have wide plateau areas. Steep, precipitous peaks tend to have less of a snow cover, because the snow is unable to pile up on the steep slopes. If a highland area is dissected into many steep slopes, avalanches will redistribute snow to the valleys, and glaciers will be confined to these gentler slopes. If a valley glacier is continuously fed by an avalanche chute, it may reach lower altitudes than the local transient snowline. In a valley, a glacier will be shaded from direct solar radiation. Shading from local topography has a major impact on the local snowline. A tall mountain will shade the equator-facing slope of a shorter one, allowing the snowline to decrease in altitude, and snow to accumulate into glaciers. (Ostrem 1966; Porter, 1975; Sugden and John, 1975).
Continentality
The position of a mountain range on a continent with respect to an ocean, especially in the mid-latitudes, has a great influence on the climate of the range. Tall, north-south trending coastal ranges generally occur on the western side of continents, where they are perpendicular to the prevailing westerly winds. These moisture-laden, maritime winds deliver great precipitation to the western slopes, while the ocean influence keeps the climate moderate. The result is that the climatic snowline tends to be lower on coast ranges than on the interior ranges. In an interior location, relatively less snow coupled with warmer summer temperatures causes increased ablation. Interior mountains also have fewer glaciers than the coastal ranges because of these factors (Price, 1981; Sugden and John, 1979).
Aspect
The orientation of the ground surface with respect to incoming solar radiation, or aspect, is particularly important on the local scale, in areas of marginal alpine glaciation, and in the temperate and sub-polar latitudes (Evans, 1969; Sugden and John, 1976). The influence of aspect in the Northern Hemisphere results in glaciers being oriented preferentially to the north, where they are sheltered by their host mountains from direct solar radiation. The opposite situation is of course true in the Southern Hemisphere, where the south side is the shady side. While in some cases glaciers have shown a strong preference for north-facing slopes, other situations occur where glaciers are found in every direction, but the biggest and most extensive glaciers are found on the north faces. Smaller glaciers seem to be particularly impacted by aspect, while large glaciers tend to be largely azonal if they have a reliable and generous snow supply, and are situated within a steep valley. Under these conditions, a glacier may extend into a warmer, sunny zone (Evans, 1969; Gilbert, 1904; Temple, 1965).
Orientation of a slope, in addition to the gradient of that slope, affect the surface receipt of incoming solar radiation (Geiger, 1965). A sloping surface facing the sun will receive much more solar radiation than if that same surface were flat. On a flat surface, the rays of the sun are hitting at a much smaller angle, causing the rays to spread, where they would be concentrated in a smaller area if the angle were larger.
There is also an east-west asymmetry with respect to glacier size and snowline elevation. Glaciers tend to orient themselves preferentially to the east. There are two reasons for this asymmetry. In the morning, the sun’s rays hit the east side of mountains. During this time, there is excess moisture on the earth’s surface, due to the cooling of the air during the night, which causes condensation and dew to form. Because of this moisture, the sun’s energy is used mainly for evaporative purposes. In the afternoon when the sun is hitting the western slopes, the excess moisture is largely gone, and solar radiation is used to heat the earth’s surface. During this time, ablation begins to occur (Price, 1981). The other reason for this uneven distribution is the prevailing wind. As was stated earlier, the mid-latitudes are dominated by westerly winds. These winds efficiently redistribute snow from the west to the east side of the mountain ranges they traverse. This piling up of snow leads to the genesis and enlargement of glaciers, as well as the lowering of the snowline. On coastal mountains, these factors may be counterbalanced by the orographic effect, and so this preferential orientation of glaciers may be less pronounced, or altogether absent. However, in the Northern Hemisphere there is generally a preference of glaciers and lowered snowlines towards the northeast slopes (Derbyshire, 1968; Gilbert, 1904; Temple, 1965).
Study area
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The Cascade Range is a north-south oriented range running along the Pacific Coast from Northern California into southwestern Canada. Because of the high altidudes reached by these young volcanic mountains, the range acts as a climatic barrier to the moist, westerly winds blowing off the Pacific Ocean. The western slopes are wet, in some places precipitation reaches over 150 inches, and in heavy winters the transient snowline reaches below 2000 feet. Because of this moist climate, vegetation is thick with Douglas fir, while on the dry, lee side of the mountain, the vegetation is mainly scrub, grasslands, and pine forest (Harris, 1976). The North Cascades of Washington presently contain an estimated 700 glaciers (Pelto, 1996; Post et al., 1971). This large concentration of glaciers can largely be attributed to the moist climate and northern latitude. The North Cascades are part of the North Cascade subcontinent, a small continent that was separated from North America until the Eocene, about 50 million years ago (Alt and Hyndman, 1984; Easterbrook ,1975). |
| Figure 2: The Cascade Range. From Harris, 1976. | Mount Baker is the tallest peak in the study area, at 10,778 feet (3285 m) above sea level. Mount Baker first erupted during the Pleistocene, long after the initial volcanic activity that created the North Cascades, and remained active into the Holocene. |
| An enormous ice cap covers all sides of Mount Baker, which contains ten major glaciers. Studies of these glaciers and of others in the North Cascades have found that they are presently retreating, following the global trend in response to warming of the climate (Harper, 1990; Sapiano et al., 1998; Spicer, 1989). |
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| Figure 3: North Cascades National Park. From the U.S. National Park Service. |
Methodology
The creation and analysis of Digital Elevation Models (DEM) within a GIS is a widely used technique in glacial and periglacial research (Allan, 1997; Bocker, 1996; Christiansen and Humlum, 1993;. Dickau, 1993; Gonzalez, Ortigosa, Marti, and Garcia-Ruiz, 1995; Walsch, et al. 1994). DEMs are useful not only for the elevation data that they provide, but also because slope and aspect models can be derived from them.
In the present study, a DEM of a region in the North Cascades was used to analyze the influence of aspect on the climatic snowline. The permanent ice caps of the Concrete Quadrangle, a 1:100,000 USGS topographic map were digitized, then overlain on a 1:250,000 USGS DEM within Arcview GIS software. For each snowcap, the lowest elevation of the snowline was determined by studying contour lines derived from the DEM. Each of these elevations was marked with a point on the map. These points were then overlain on an aspect model derived from the DEM, and each point’s azimuth was identified from the aspect grid. The azimuths were put into a table and imported in GEO-ORIENT software in order to produce a compass rose diagram. This program also calculated the vector mean for the azimuths.
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Figure 4: Snowcaps overlayed on the DEM. |
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Figure 5: A point is placed marking the lowest elevation for each polygon. |
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Figure 6: The point theme is overlayed on the aspect grid. |
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| Figure 7: GEO-ORIENT rose diagram of the azimuth
data. The arrow at the
top indicates the vector mean. |
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Results
It is evident from the rose diagram and the vector mean of 11 degrees that the snowline of the North Cascades is generally lowest on the northeast side of the mountains. This data can probably be interpreted to mean that glaciers in this region preferentially develop or at least grow to larger sizes on the northeast slopes. However, it cannot be determined for certain given the scale and resolution of the map and study area. The careful examination of aerial photography and extensive fieldwork could provide a verification of this hypothesis.
It is apparent that the snowline is lower on the east, rather than the west-facing slopes, indicating that the orographic effect is not counterbalancing the strong influences of the westerly winds and the position of the sun on the elevation of the snowline, even in this moist, maritime climate.
Summary and conclusion
The elevation of the alpine snowline and glaciation limit are mainly controlled by temperature, precipitation, latitude, altitude, aspect, and slope and topography. These factors influence each other as well as being direct controls of the spatial distribution of permanent snow. The importance of each of these factors changes at every scale. The influence of latitude is most important on a global scale, while slope and aspect play the greatest role at the local scale. The climatic snowline of the North Cascades is a good example of the regional effects of aspect, showing a strong trend to the northeast as its lowest average altitude.
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