SEASONAL REORGANIZATION OF SUBGLACIAL DRAINAGE INFERRED FROM MEASUREMENTS IN BOREHOLES

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.