Black Rapids Glacier is a surge-type glacier located in central Alaska Range. During its last surge in 1937, the terminus rapidly advanced toward the Richardson Highway. Large seasonal changes in surface speed have also been observed each summer (up to 400%) over the past twenty years. By studying the causes for these changes in speed, which are due primarily to changes in basal motion, we hope to be able to characterize the nature of ice/rock interface and describe the interrelations between this interface and seasonal surface water inputs. These studies may also help in understanding the surge mechanisms of this glacier during its next surge.

During the period of May to July, 1993, UAF and University of Washington researchers collected data related to the spring speedup of the glacier. The velocity of several surface markers along the glacier was measured twice daily during this period. This data set allows us to study the effects of changing basal conditions down a length of the glacier. Seismic reflection measurements of the bed were made every day, along with daily ice radar measurements by UW researchers. These data are used to directly measure changes at the bed which can then be correlated with changes in the surface speed. In addition, the river emanating from the terminus was monitored continually throughout the summer. Stream measurements included stage and dye tracing, which provides us with an idea of how much water was stored at the bed and for how long, as well as electrical conductivity and turbidity, which provide important clues as to nature of the basal hydrology.

The analysis of this data is still ongoing. There is a wealth of data to analyze, each with its own unique thread of evidence which we are trying to weave into a complete picture of basal hydrology. The analysis is further enhanced by measurements related to the draining of several ice marginal lakes during our summer field season. These lakes drained subglacially, producing large increases in surface speed and changes in the stream record. By comparing the timing and discharge of these events with our measurements of glacier speed and changes at the bed and stream, we are able to test hypotheses regarding mechanisms of the spring speedup and basal motion in general.

NSF-OPP Grant OPP91-22783: Scientific personnel; Matt Nolan, Oakley Cochran, Keith Echelmeyer, and Will Harrison (Geophysical Institute) and Tony Gades and Charlie Raymond (University of Washington)