Structural control of englacial conduits in the temperate Matanuska Glacier, Alaska, USA

Fourteen englacial conduits were mapped within 2 km of the terminus of the temperate Matanuska Glacier, Alaska, USA, to ice depths of 65m using speleological techniques. Detailed threedimensional maps of the conduits were made over 3 years to characterize conduit relationships with glacier structural features and to track conduit evolution through time. All conduits consisted of single unbranching passages that followed fractures in the ice. All conduits were either too constricted to continue or became water-filled at their deepest explored point and were not able to be followed to the glacier bed. Conduit morphology varied systematically with the orientation of the glacier principal stresses, allowing them to be categorized into two broad classes. The first class of conduits were formed by hydrostatic crevasse penetration where a large supraglacial stream intersected longitudinal crevasses. These conduits plunged toward the glacier bed at angles of 30–408. The second class of conduits formed where smaller streams sank into the glacier on shear crevasses. Many of these conduits changed direction dramatically where they intersected transverse crevasses at depth. These results suggest that the conduits observed in this study formed along fractures and, over their surveyed length, were not affected by gradients in ice overburden pressure.