Beneath the upper part of the ablation area of Storglaciären, northern Sweden, the glacier bed is overdeepened. The overdeepening has a closure of ∼60 m.
Electrical resistivity measurements in bore holes indicate that a layer of till, ∼0.5 m thick, lies between the glacier and the bed in this overdeepening in at least one place; we infer that this till layer underlies the glacier throughout the better part of this overdeepening.
Of about 40 holes drilled to the bed in this overdeepening between 1982 and 1987, ∼70% “drained” when the drill tip had penetrated between ∼40 and ∼95% of the glacier thickness. When measured after completion of drilling, however, the water levels were only 20-45 m below the glacier surface, and were relatively stable, varying only 5–10 m, say, in a period of several days. These levels correspond to basal water pressures on the order of 80–90% of overburden pressure. In contrast, water levels further down-glacier vary up to 50–100 m diurnally during good weather. This brings us to the first of two questions:
What sort of “dam” maintains water levels in the overdeepening several tens of metres above the low point of the riegel at the down-glacier end of the overdeepening?
Tracer tests indicate that at least three of the bore holes that “drained” intersected englacial drainage channels at the levels where the drill tip stood when the water level dropped from the surface. Other tracer tests indicate that water passing through the overdeepening appears at the terminus in a stream that carries little sediment. In contrast, tracer tests further down-glacier indicate that water disappearing down moulons over the riegel appears in another stream that carries a lot of sediment. Thus, water flow through the overdeepening is inferred to be englacial. This raises the second question:
Is the water flow englacial because the till layer is present, and the till layer squeezes into incipient subglacial channels, closing them? If so, why is the till layer present?
Alternatively, is the till layer present because the water flow is englacial, and thus there is no flushing along the bottom to remove the products of erosion? If so, why is the drainage englacial? Is it because water flowing up an adverse slope must warm to remain at the pressure-melting point? If slopes are steep enough, such warming can require refreezing which would tend to close subglacial channels.
