TEMPORAL SWITCHING BETWEEN ENGLACIAL AND SUBGLACIAL DRAINAGE PATHWAYS- DYE TRACER EVIDENCE FROM THE HAUT-GLACIER-DAROLLA, SWITZERLAND

Thirty-four dye tracing experiments conducted from a moulin (m3Cf), lo cated 1.5 km from the snout of the Haut Glacier d'Arolla, Switzerland, during the summers of 1989-1991 reveal two distinct drainage paths be tween the moulin and the glacier snout. In July, drainage appears to b e predominantly via a hydraulically efficient channelised drainage sys tem which results in dye breakthrough curves with velocities of >0.3 m s(-1) and dispersivities of <10 m. In August, drainage is via a more hydraulically inefficient distributed system which results either in v elocities of <0.2 m s(-1) and dispersivities >10 m or in zero dye reco very. One injection conducted in mid July 1991 produced a double peake d breakthrough curve, indicating simultaneous drainage through both sy stems. The observed behaviour is opposite to that displayed by injecti ons made at over 50 other moulins on the glacier, in which channelised flow replaces flow through a distributed system as the melt season pr ogresses. It can be explained if moulin m3Cf intersects an englacial c onduit which conveys water rapidly to a major subglacial channel in Ju ly when the distributed system is poorly developed and water pressures are high, but which is abandoned later in the year when increased gla cier bed separation has expanded the drainage capacity of the distribu ted system. Dilution and retardation of dye within this enlarged distr ibuted system can account for the frequency of zero recoveries from th e August dye injections.