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

Discharges of water, sediment, and dissolved impurities from Variegated Glacier, Ala ska, were monitored in the early summers of 1980 and 1981 during the occurrence of mini-surg es. Sea sonal trends, weat her-related eve nts, and diurnal variat ions similar to behavior of other temperat e gl acier streams were found. The prin cipal effect in the stream associated with mini-surge occurrence was a brief discharge of extremely turbi d 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 locat ion on the upper glacier known from oth er meas ure­ ments. The mean water velocity in the hydraulic system over the int erven ing distance is thereby determined (0.3 m S-I). The mean water velocity, together with the water discharge ("'16 m3 S-1 at the terminus), places con straints on the distribution of water velocity u and total cros s-sectional area AT of the flow paths along the gl acier base. This leads to the con clusion that within the zone of mini-surge occurrence in its unpe rturbed state: u is about 0.1 m S-1 or possibly less; AT is about 10 2 m2 or possibly more, and it must be divided into a very large number of small passag eways, be blocked by constrict ions, or both. The total water cro ss-se ction corresponds to a layer 0.1-{).2 m thick when spr ead unifo rmly over the glacier width. The water velocity is clos e to or less than the propa gation velo city of the mini-s urges. Bet ween the zone affected by mini-surges and the stream, a dyna mically less active lower section of the glacier is proba bly underlain by a small number of conduits, in which the water velocity may be very high (�2 m S-I). Water di scharge follo wing the mini­ surges puts an upper limit on water-storage changes ass ociated with the anomalous ice motion.