Annals of Glaciology 13 1989 @ International Glaciological Society THE STORAGE OF WATER IN, AND HYDRAULIC CHARACTERISTICS OF, THE FIRN OF SOUTH CASCADE GLACIER , WASHINGTON STATE, U.S.A. by Andrew G. Fountain* (Project Office-Glaciology, U.S. Geological Survey, Tacoma, WA 98402, U.s.A.) ABSTRACT The porosity and water saturation of the firn of South Cascade Glacier were measured in order to determine both the volume of water stored in it and the significance of this water content for the water volume stored in the glacier. The distance to water below the firn surface was found never to be greater than 4 m, and the average thickness of the water-saturated layer was estimated to be 1.25 m. The average firn porosity was 0.15, the water saturation was 0.61, and the total volume of water stored in the firn was approximately 1.78 x 10 5 m 3 representing about 12% of the total spring storage. The water table was found to exhibit a pattern of diurnal fluctuation which starts in late June or early July, indicating that melt water from the accumulation zone may pressurize the bed at diurnal frequencies. The depth-averaged permeability was found to be 1.5 x 10- 5 m / s, a value which compares favorably with those from other studies. INTRODUCTION The flow of water through a glacier is a complicated process in which many components are poorly understood. One aspect of this process is the flow of water through the firn layer located in the accumulation zone . In temperate glaciers, the firn functions as a thin, unconfined aquifer that stores water and retards its movement to the base of the glacier. This effect is important in determining the magnitude of glacially stored water during a summer season (Tangborn and others, 1975; bstling and Hooke, 1986) and in quantifying the water flux from the glacier surface to its interior. Knowing the water flux is particularly important for time-dependent models of the glacial hydraulic system. The intention of the present study is to determine the volume of water seasonally stored in the firn layer of a temperate glacier, to estimate its significance for the volume of water stored in the glacier as a whole, and to quantify firn permeability. This report describes the results and implications of several field experiments at South Cascade Glacier, Washington State, U .S.A. EARLIER WORK The presence of a water table at a depth of between 5 and 40 m in the accumulation zone of temperate glaciers is a common phenomenon (Sharp, 1951; Schommer, 1977; Ambach and others, 1978; Oerter and Moser, 1982). In general, the distance to water from the glacier surface increases with increasing distance up-glacier from its equilibrium line (Ambach and others, 1978). Further, as one would expect from ground-water hydraulics, the slope of the water table roughly approximates to the large-scale *Prese nt address: U.S. Geological Survey, P.O. Box 25046, MS-412, Denver Federal Center, Lakewood, CO 80225, U .S.A . surface slope of the glacier, although the two slopes are not parallel (Lang and others, 1976; Schommer, 1977, 1978). The distance to, and thickness of, the water table also varies with its proximity to hydrologic sinks such as crevasses, a fact which is indicated by the lack of water in bore holes near crevasses (Lang and others, 1976; Schommer, 1977). In general, the water table rises in the spring and falls in the autumn, with little or no variation in the winter (Lang and others, 1976; Schommer, 1977). Such characteristic seasonal fluctuations have been explained by Schommer (1978) and Ambach and others (1981), using numerical and analytical models res pectively, as an expected consequence of a porous medium subjected to seasonal input variations . Sudden and otherwise unexpected drops in water levels are commonly associated with the opening of nearby crevasses (Schommer, 1977). Diurnal variations in the water table have been observed in some bore holes (Ambach and others, 1981) but not in others (Lang and others, 1977; Schommer, 1977) despite the fact that the water table was at approximately the same distance below the glacier surface in all cases. To make a quantitative estimate of water storage and of flow velocity in the firn, a knowledge of its porosity and permeability is requir ed in addition to the thickness of the water layer. The poro sity, the ratio of void volume to sample volume, was used by Schommer (1978) as an adjustable parameter in numerical calculations with a magnitude of 0.20-0.30. Similarly, Ambach and others (1981) used a porosity that varied linearly with depth, with values ranging from 0.50 at the firn s urface to 0. 10 at the firn-ice transition , independent of actual depth below surface of the water layer. Oerter and Moser (1982) estimated a porosity value of 0.15, with a water saturation of 0.5. Permeability is usually estimated using a standard ground-water aquifer tes t. The change in water level as a function both of time and of dis tance from the pumped hole is related to the hydraulic perm eab ility of the medium (Freeze and Cherry, 1979). Re sults of tests completed by the present and previous investigators are shown in Table I. Although previous stud ies meas ured the depth to the water table and estimated firn permeability, there are no reports of porosity measurements or thickness of the water layer. Therefore, the volume of water stored in the firn cannot be calculated. In addition, permeability has been measured at only one location on each glacier, although it is known that its value can range over one or two orders of magnitude for earthen aquifers (Freeze and Cherry, 1979). For this reason a routine for spatial measurement of permeability values would be desirable . STUDY SITE South Cascade Glacier (Fig. I) is a small valley glacier located in the Cascade Mountains in the north-we s tern corner of the U.S.A. It ranges in elevation from 1630 to 2100 m a.s.I., and its equilibrium-line elevation is at approximately 1870 m a.s.l. The area of the glacier is about 69 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0260305500007667 Downloaded from https://www.cambridge.org/core. 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