HYDROLOGY OF A SEGMENT OF A GLACIER SITUATED IN AN OVERDEENING, STORGLACIAREN, SWEDEN

Tracer experiments, and water-level observations made while drilling 4 7 boreholes in an overdeepened section of Storglaciaren, have demonstr ated that nearly all of the water passing through this part of the gla cier moves in englacial conduits. Much of the viscous energy dissipate d by subglacial water flowing up an adverse bed slope out of such an o verdeepening may be needed to warm the water to keep it at the pressur e-melting point. If the adverse slope is sufficiently steep, freezing may occur within the conduits. The possibility for enlargement of cond uits by melting is thus limited and water pressures become high. We in fer that this, combined with possible blocking of conduits by freezing , forces the water to seek englacial pathways. The frequency with whic h englacial conduits are encountered during drilling suggests that the re are several hundred of them-in any given cross-section of the glaci er. Consequently, each must carry a small fraction of the total discha rge, say approximately 10(-3) m3 s-1. Tracer experiments suggest that flow rates in these conduits are < 10(-1) m s-1, so conduit cross-sect ional areas must be approximately 10(-2) m2, a size that is consistent with video observations in boreholes. The observed mean hydraulic gra dient through the overdeepening is approximately 0.04. If the conduits were of uniform cross-sectional area, the roughness implied by these figures would be unreasonably high and water pressures in them would b e lower than observed. Thus, we hypothesize that conduits are locally constricted to only a small fraction of their average cross-sectional area.