1. Departments of Geology and Geography, Portland State University, and
2. Cleveland High School, Portland, Oregon 97202, USA
3. Department of Physics, St Olaf College, Northfield, Minnesota 55057, USA
4. Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91, Stockholm, Sweden

Correspondence to: Andrew G. Fountain¹ Correspondence and requests for materials should be addressed to A.G.F. (Email: andrew@pdx.edu).

Understanding the flow of water through the body of a glacier is important, because the spatial distribution of water and the rate of infiltration to the glacier bottom is one control on water storage and pressure, glacier sliding and surging, and the release of glacial outburst floods^{1, 2, 3}. According to the prevailing hypothesis, this water flow takes place in a network of tubular conduits^{4, 5}. Here we analyse video images from 48 boreholes drilled into the small Swedish glacier Storglaciären, showing that the glacier's hydrological system is instead dominated by fractures that convey water at slow speeds. We detected hydraulically connected fractures at all depths, including near the glacier bottom. Our observations indicate that fractures provide the main pathways for surface water to reach deep within the glacier, whereas tubular conduits probably form only in special circumstances. A network of hydraulically linked fractures offers a simple explanation for the origin and evolution of the englacial water flow system and its seasonal regeneration. Such a fracture network also explains radar observations that reveal a complex pattern of echoes rather than a system of conduits. Our findings may be important in understanding the catastrophic collapse of ice shelves and rapid hydraulic connection between the surface and bed of an ice sheet.

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