Paper:  Borehole water-level variations and the structure of the

subglacial hydrological system of Haut Glacier d'Arolla, Valais,
Switzerland, Hubbard et al. 1995

Summary by Katie

One of the main points of discussion and sources of frustration to
many group members was the authors' selective use of borehole data and
the short time period analyzed. Bernard suggested the highlighted data
contained their clearest story, and they left out other data so as not
to confuse the reader. Twila commented that, for a more complete
picture of what happened throughout the study, they should have
included all of the results and an explanation as to why they excluded
certain points from their analyses.

The limited data presented in this paper lead others to wonder about
the broader conclusions we can reach, both for this particular glacier
and subglacial processes in general. Ed and Marc agreed that the most
robust conclusion is the presence of a diurnal, lateral flow of water
for ~70 m on either side of a subglacial channel, characterized by
high (1-10g/L) suspended sediment concentrations in the return flow.
How this fits into the bigger picture of subglacial hydrology is not
well known. Many commented that measurements of water turbidity at the
terminus would be a useful complement to this dataset. The permanency
of the channel was also questioned, and whether other "discrete"
channels characterize this system.

Discussion also focused on interesting observations presented in the
study. We addressed the question of why there is such a strong
asymmetry in the water pressure measured in the boreholes near the
channel (see Fig. 5). Ed explained that the total water pressure is
the same for flows into and out of the channel, but the relative
hydraulic conductivity changes. The return flow is slower because the
channel-proximal cavities created during the rapid outflow close
quickly, resulting in slower drainage back into the channel. Rachel
pointed out that GPS observations would be very useful to measure the
surface changes during the same time period.

We also talked about the removal of fine-grained particles during the
return flow. The group questioned whether the sediment-laden, diurnal
pulse of water is evacuated all the way to the terminus, and what
effect this would have on the overall subglacial storage of fines.
Adam C. brought up the importance of understanding the net result of
these flows integrated over time and space, and that fines are
ubiquitous in glacial till (thereby not completely transported out of
the system). Bernard explained that the fine particles are products of
abrasion, which are then stored in small subglacial cavities. These
cavities are drained with enough frequency (presumably through
different channel paths) in order to sustain a continuous flow of
turbid water. Bernard also added that the fines are likely not erosion
products of already produced till.