1966 HYDROLOGIC YEAR
Gulkana Glacier was visited in late September 1965, when preliminary ice-balance measurements were made at two sites (table 5). During the summer (May-September 1966) the snow balance was measured, an additional balance site established, a hut constructed, and a streamflow gaging station installed (pl. 5A). Continuous streamflow data, however, were not produced until almost the end of the hydrologic year, so none are reported here; nor are any continuous temperature or precipitation data available. These data will appear in reports covering subsequent hydrologic years.
TABLE 5.Instrumentation at Gutkana Glacier during the 1966 hydrologic year
The weather during the 1966 hydrologic year at several nearby low-altitude weather stations had no outstanding variations from a normal pattern, and the glacier balance was apparently in the normal range. The large new snow accumulation during August and September 1966 occurred during the time when the maritime coastal regions were receiving more than usual precipitation. One measurement made on Gulkana Glacier on September 26, 1965, indicates that roughly 0.10 m water equivalent of new snow, the initial snow balance, 0(s), was present on the glacier at the beginning of the 1966 hydrologic year.
The snowpack in the basin on May 18, 1966 (pl. 5B), averaged 0.82 m water equivalent (m(s), table 6). This was 5-10 times more than the winter snowfall recorded at several weather stations in the Copper River basin to the south and the Tanana River valley to the north. The relation of late-winter snow balance to altitude (pl. 5C) cannot, however, be interpreted to be the same as the change of precipitation with altitude. Some of the increase with altitude is caused by storms which rain on the lower elevations but snow higher up. In addition, a large part of the snow which falls high in Gulkana Glacier basin is blown away by strong south winds; this snow either evaporates or is redeposited on Canwell Glacier to the north. Little or no snow is blown into Gulkana Basin by these same winds because no high windblown mountains are present immediately to the windward south side of the basin. The average snowpack density on May 18 was about 0.36 megagrams per cubic meter.
TABLE 6.Snow and ice balances, Gulkana Glacier, 1966 hydrologic year
[Parameter values and errors in meters except where indicated, Date: Hydrologic year, Oct. 1, 1965 (t0), to sept, 30, 1966 (t1)]
The snowpack reached its maximum balance, x, about May 20, 1966, at the lowest point in the basin; maximum balance was probably reached about July 15, 1966, high in the basin. Possibly the highest areas accumulated snow continually throughout the year. About 55 percent of the snowpack measured May 18 on the glaciers did not melt, and this residual snow was identifiable as firn in September 1966. High in the eastern cirques, summer avalanches and storms caused considerable accumulation. Small patches of firn remained on the glacier at altitudes as low as 1,600 m. New snow definitely began accumulating by August 15 at 1,940 m and by September 15, 1966, at 1,400 m.
Ablation of glacier ice began early in May 1966 from snowfree (windblown) morainal ridges low on the glacier (pl. 5A). The melting became most rapid in July and continued until the ice became fully snow covered in late September 1966 (pl. 5D). Ice ablation measurements were made on September 23, after the ice was snow covered. Some ablation of the previous year's firn occurred in July and August, and this is included as glacier-ice ablation. Near the glacier terminus, a maximum ablation of 4.0 m occurred on bare ice; however, a 10-20 cm layer of supraglacier moraine reduced the annual ice ablation locally to about 2.5 m. Isolated areas of ice ablation occurred up to an altitude of 2,300 m. The amount of glacier-ice ablation exceeded the formation of firn, so the annual firn and ice balance was -0.21 m (pl. 5E, table 6).
Heavy snowfall in August and September covered the entire basin with new snow by September 20, 1966 (pl. 5D). From measurements taken September 23, it is estimated that the basin had an average final snow balance, 1(s), of 0.30 m at the end of the hydrologic year. This heavy snow accumulation exceeded the initial snow balance and more than made up for the basin's negative firn and ice balance, a(fi), of -0.19 m; as a result the basin gained 0.03 m storage, a, during the 1966 hydrologic year (table 6).
The continuous formation of kettle holes and the exposure of ice after mudflows indicates that ablation from the ice-cored moraine was occurring. The amount of melting (pl. 5E) was estimated to be greater than 1 m in localities having 20 percent bare ice exposed and 0.1 - 0.5 m where the ice is buried below alluvium and till.
The terminus of the glacier is stagnant and consists of nearly level ridges of ice. Ablation of some of this ice caused a terminal retreat of about 44 m. The glacier has retreated 530 m since 19S4 and about 2 km since 1910 (Reger, 1964, pl. 1), an average retreat rate of 36 m per year.
Precipitation and Runoff
The annual precipitation and runoff during the 1966 hydrologic year are estimated to be from 1.5 to 2 m on the basis of the data for snow and ice balance and on an estimate of 0.5 m of additional ungaged precipitation. The precipitation and runoff from the low forested regions of interior Alaska are only 10-20 percent of the estimated values for Gulkana Basin; therefore, the glacier-covered Alaska Range contributed an important part of the water runoff from the area.
Last Updated: 28-Mar-2006