Jakobshavns Isbra; is a fast-moving outlet glacier draining a significant part of the Greenland ice sheet. This glacier extends nearly 100 km into the ice sheet as a well-defined ice stream. The lower glacier moves at speeds of over 7 km/year below the grounding zone, with a gradual decrease in speed inland into the ice sheet. This glacier moves at these high speeds continuously. Such high speeds are comparable to average velocities observed on surge-type glaciers during periods of peak surge, when basal sliding has been shown to be the dominant flow mechanism. An important question is whether or not the high speeds on Jakobshavns Isbrae are due to a large sliding contribution or to some other mechanism such as an ice-flow instability near the base, and, if it is sliding, what causes these high slip rates?

A major field program was undertaken on Jakobshavns Isbra: during 1984–87 to address these questions and others relating to ice-stream dynamics. Measurements have included spatial and temporal variations of the surface-velocity field, mass balance, ice temperature, seismic activity, calving rate and terminus position, ice-fabric analysis, and ice thickness.

Measurement of surface velocity throughout the year shows no seasonal variation in velocity. Production of large amounts of melt water at the surface in the ablation zone (lower 75 km of the ice stream) does not appear to influence the speed directly. Calculations show that basal shear stresses along the ice stream are large. If there is a basal zone of favorably oriented ice (and therefore a weak layer), then ice deformation under the large stresses may account for the fast motion in the ice stream rather than basal sliding or deformation of basal debris, as has been proposed for the Antarctic ice streams.

At and below the grounding zone, short-interval

surveys show that the velocity varies by as much as 35% with the level of the tide in the fjord. The phasing of this harmonic speed variation with the tide shows that basal sliding is occurring at the grounding zone and that this sliding is directly affected by subglacial water.

Observations of ice temperature strongly show the effects of melt-water refreezing in the percolation zone. These and other interesting results from the extensive field program are discussed, giving further insight into the dynamics of this fast-moving ice stream.