Short-term velocity and water-pressure variations down-glacier from a riegel, Storglaciaren, Sweden

During the 1991-94 summer field seasons, time-correlated measurements of water. pressure and surface speed were made over and down-glacier from a major riegel on Storglaciaren, Sweden. Measurements were made at sub-hourly time-scales in order to discern details in the diurnal cycle. Large water-input events, typically associated with rain storms, produced coherent, lagged surface-velocity responses that could be understood in terms of till deformation or decoupling, and these have been discussed elsewhere. The consequences of smaller diurnal water-pressure events were more enigmatic, in that acceleration of ice flow generally preceded the onset of the local water-pressure rise. From consideration of these data and other work done on the hydrology of Storglaciaren, we infer that thr ice in this area is generally pushed from behind via a relaxation in extensional strain across the riegel. Hence, accelerations occur in response to increases in water pressure that occur up-glacier and that precede local water-pressure rises. In addition, following a period of large storm events, surface speeds became more spatially coherent and were in phase with the diurnal water-pressure cycle. This suggests that the large water-pressure events lead to a spatially more homogeneous subglacial drainage system. Sliding laws need to take into of the subglacial drainage system.