Discharges of Turbid Water during Mini-Surges of Variegated Glacier, Alaska, U.S.A.

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Abstract
Discharges of water, sediment, and dissolved impurities from Variegated Glacier, Alaska, were monitored in the early summers of 1980 and 1981 during the occurrence of mini-surges. Seasonal trends, weather-related events, and diurnal variations similar to behavior of other temperate glacier streams were found. The principal effect in the stream associated with mini-surge occurrence was a brief discharge of extremely turbid water. The turbidity is assumed to be introduced into the basal hydraulic system by initiation of the fast motion of a mini-surge at a time and location on the upper glacier known from other measurements. The mean water velocity in the hydraulic system over the intervening distance is thereby determined (0.3 ms−1). The mean water velocity, together with the water discharge (≈16 m3s−1at the terminus), places constraints on the distribution of water velocityuand total cross-sectional areaATof the flow paths along the glacier base. This leads to the conclusion that within the zone of mini-surge occurrence in its unperturbed state:uis about 0.1 ms−1or possibly less;ATis about 102m2or possibly more, and it must be divided into a very large number of small passageways, be blocked by constrictions, or both. The total water cross-section corresponds to a layer 0.1–0.2 m thick when spread uniformly over the glacier width. The water velocity is close to or less than the propagation velocity of the mini-surges. Between the zone affected by mini-surges and the stream, a dynamically less active lower section of the glacier is probably underlain by a small number of conduits, in which the water velocity may be very high (≥ 2 m s−1). Water discharge following the mini-surges puts an upper limit on water-storage changes associated with the anomalous ice motion.