Observations of englacial water passages: a fracture-dominated system Andrew G. FOUNTAIN, 1 Robert B. SCHLICHTING, 2 Peter JANSSON, 3 Robert W. JACOBEL 4 1 Departments of Geology and Geography, Portland State University, PO Box 751, Portland, OR 97207-0751, USA E-mail: fountain@pdx.edu 2 Cleveland High School, 3400 SE 26th Avenue, Portland, OR 97215, USA 3 Department of Physical Geography, Stockholm University, SE-106 91 Stockholm, Sweden 4 Saint Olaf College, 1520 St Olaf Avenue, Northfield, MN 55057, USA ABSTRACT. To test models of the hydraulics and geometry of englacial conduits, 48 holes (3900m of ice) were drilled into Storglaciaren, Sweden, in search of conduits. About 79% of the holes intersected a hydraulically connected englacial feature. A video camera was used to examine the features and measure local water-flow rates. Because of the extremely clear ice that surrounded most features, their geometry could not be discerned. Of the remaining features, 80% (36) were fracture-like, 16% (6) were of complex geometry, and 4% (2) exhibited a conduit-like geometry. The fracture-like features exhibited steep plunges (708), narrow openings (40 mm) and slow water-flow speeds (10 mm s –1 ). We argue that these fracture-like features are indeed englacial fractures of unknown origin. The depth to fractures intersection varied from near the glacier surface to 96% of local ice depth, with a maximum depth of 131m. Few hydraulically connected fractures exhibited water motion, indicating some preferential flow pathways exist. We found one ‘traditional’ englacial conduit after an intentional search in a field of moulins. These results suggest that englacial water flow is conveyed through a ubiquitous network of fractures and that conduits are relatively rare. INTRODUCTION The presence of water at the glacier bed profoundly affects the movement of alpine glaciers (Kamb and others, 1985; Iken and Bindschadler, 1986) and polar ice streams (Alley and others, 1987; Echelmeyer and Harrison, 1990; Kamb, 1991). Water stored within and under a glacier can also result in outburst floods (Clarke and Waldron, 1984; Anderson and others, 2003). For these reasons, most studies on the internal hydraulics of glaciers have focused on the subglacial regime. In temperate alpine glaciers and ice caps, the nature of the englacial hydraulics is also important because englacial passages route water from the glacier surface to the bed (Fountain and Walder, 1998). The presence or absence of englacial passages controls the spatial distribution of water at the bed, which may affect local ice movement. These passages can also temporarily store water (Jacobel and Raymond, 1984), which may be important to the development of outburst floods from glaciers. Englacial water passages have been thought to be semicircular in cross-section, and extending laterally for many tens of meters (Ro ¨thlisberger, 1972; Shreve, 1972). Such passages are commonly observed at the glacier margins and within moulins (Holmlund, 1988). In theory, the radius of the passage cross-section results from the balance between the inward creep of ice and the outward melting of the ice walls due to the frictional heat produced by the flowing water. The processes governing the steady-state geometry and water pressure of such englacial conduits were first modeled by Ro ¨ thlisberger (1972) and Shreve (1972). Despite the generally accepted theoretical models of englacial conduits (Ro ¨thlisberger, 1972; Shreve, 1972), few direct measurements of englacial water passages exist against which the models may be tested All of the available data have been derived from studies of subglacial processes whereby interesting englacial features were noticed in holes that had been drilled to the bed. Hodge (1976) found englacial voids with vertical extents of about 0.1 m, and Raymond and Harrison (1975) found millimeter-scale passages. Whether these voids and passages were part of an active hydraulic system was unclear. Video cameras lowered into holes drilled to the glacier bed (Pohjola, 1994; Harper and Humphrey, 1995; Copland and others, 1997) revealed that englacial voids were fairly common, of which only a few were described as a conduit. Diameters of the inferred conduits averaged about 0.1 m (Hooke and Pohjola, 1994). The estimated flow speeds were slower than expected for these conduits, ranging from 0.01 to 0.1 m s –1 . During the summers of 2001–03 we conducted a field study at Storglacia ¨ren, Sweden, to intercept englacial conduits. The primary objective was to visually measure englacial conduit structure and hydraulic characteristics to directly test the theoretical models of flow in conduits. We report here on the geometry of the englacial passages we observed. Throughout this paper, the term ‘conduit’ refers only to englacial passages that conform to the trad- itional models (Ro ¨thlisberger, 1972; Shreve, 1972). We use ‘passage’ to refer to any hydraulically connected englacial feature. STUDY SITE Storglacia ¨ren (Fig. 1) is a small alpine glacier in northern Sweden with an area of about 3.1 km 2 (Jansson, 1996). The subglacial topography is well known (Bjo ¨ rnsson, 1981), with a maximum depth of about 250 m. This glacier has a long history of study, dating from 1946 when a program of Annals of Glaciology 40 2005 25