S. Gordon et al., SEASONAL REORGANIZATION OF SUBGLACIAL DRAINAGE INFERRED FROM MEASUREMENTS IN BOREHOLES, Hydrological processes, 12(1), 1998, pp. 105-133
The effect of the formation of a major subglacial drainage channel on
the behaviour of the subglacial drainage system of Haut Glacier d'Arol
la, Switzerland, was investigated using measurements of borehole water
level and the electrical conductivity and turbidity of basal meltwate
rs, Electrical conductivity profiles were also measured within borehol
e water columns to identify the water sources driving water level chan
ges, and to determine patterns of water circulation in boreholes. Prio
r to channel formation, boreholes showed idiosyncratic and poorly coor
dinated behaviour, Diurnal water level fluctuations were small and dri
ven by supraglacial/englacial water inputs, even when boreholes were c
onnected to a subglacial drainage system, This system appeared to cons
ist of hydraulically impermeable patches interspersed with storage spa
ces, and transmitted a very low water flux, Drainage reorganization, w
hich occurred around 31 July, 1993, in response to rapidly rising melt
water and rainfall inputs, seems to have involved the creation of a co
nnection between an incipient channel and a well established channeliz
ed system located further down-glacier. Once a major channel existed w
ithin the area of the borehole array, borehole water level fluctuation
s were forced by discharge-related changes in channel water pressure,
although a diversity of responses was observed. These included (i) syn
chronous, (ii) damped and lagged, (iii) inverse, and (iv) alternating
inverse/lagged responses. Synchronous responses occurred in boreholes
connected directly to the channel, while damped and lagged responses o
ccurred in boreholes connected to it by a more resistive drainage syst
em. Pressure variations within the channel resulted in diurnal transfe
r of mechanical support for the ice overburden between connected and u
nconnected areas of the bed, producing inverse and alternating pattern
s of water level response. (C) 1998 John Wiley Sons, Ltd.